What changed that made TenSoon want to run?

Question

TenSoon is a kandra, a type of sentient magical creature in Sanderson's Mistborn series, and a proud one at that. After the events in the second book of the series, he travels to his homeland, willingly, to stand trial for several offenses that he had perpetrated against his people (and to warn the kandra of a coming danger related to those incidents). There is a very strong element of honor in the kandra culture, and TenSoon would never run away from captivity. This is mentioned multiple times in The Hero of Ages before his escape. Here, from chapter 9:

He would not run, and he would not strike down one of his own. He was better than that.

And from chapter 20:

TenSoon sat in his cage.

The cage's very existence was an insult. Kandra were not like men -- even if he were not imprisoned, TenSoon would not have run or tried to escape. He had come willingly to his fate.

However, by the end of chapter 20, something has already changed. A female acquaintance of his, MeLaan, visits him in his cell. She exhorts him to fight, to rebel against the governing body of the kandra people. He responds that he would not, but then begins manipulating his captors into affording him the opportunity of escape by mentioning that his dog's body was humiliating and disgraceful to him. This ensures that when he is taken for his execution (ch 33) he has a body that is significantly faster than his captors', allowing him to outrun them and escape successfully.

So whatever change occurred, it must have happened somewhere between the beginning and the end of the 20th chapter. Something happens, or TenSoon reaches some conclusion, that changes him from the "no, I would never try to escape even if it were possible" kandra that he was, to a "less orthodox" (his words) and rebellious kandra who takes steps to create and utilize a way out of his lawful (if somewhat unjust) execution. However, I think I missed it, whatever it was.

What caused this sudden and striking change in TenSoon's character?

I understand that this change serves to emphasize the kandras' utter surprise when TenSoon begins his escape (as well as some mild surprise for the readership), but a change this dramatic should be caused by something. TenSoon has lived approximately 700 years, and humans 1/20th of his age don't change like that without strong cause.

Answer 1

He realises that more is at stake than he'd thought.

Firstly, note that TenSoon's change of heart is more gradual than you've made it sound. OK, it's still pretty quick from the point of view of a centuries-old creature, but it doesn't all happen in a single chapter. There's actually a whole series of passages which show the change step by step.

• TenSoon's original crime, of breaking Contract and telling Vin the secret of controlling kandra back in The Well of Ascension, was committed on impulse and without thinking. He accidentally, as it were, became a rebel and a traitor. Then, like a good kandra, he returned to the Homeland to confess his crime and give himself up for judgement. He didn't have to do so - no other kandra knew what he'd done - but at that point he was still too law-abiding not to.

• Later, he rationalised what he'd done, convincing himself that the kandra's duty was now to serve Vin and assist her in the inevitable conflict with Ruin. It's for this reason that he demands a public trial rather than private execution:

  By revealing his accusations in an open forum, he would earn himself an eternity of pain. Forcing a trial would be foolish, for there was no hope of vindication. His confessions had already damned him.
  If he spoke, it would not be to defend himself. It would be for other reasons entirely.
  "Judgement," he repeated, this time barely whispering.

  _{-- Chapter 2}

  Note that it may take him as much as a year even to change this far. He spends a year imprisoned in a pit before going to face judgement, and with nothing to do there but think, he has plenty of time to consider his actions and the reasons or justifications for them.

• When he comes to face judgement, he still doesn't even consider the possibility of escape.

  They didn't bind his hands; that would have been too much of an insult even for him. Kandra obeyed the Contract, even those of the Third Generation. He would not run, and he would not strike down one of his own. He was better than that.

  _{-- Chapter 9}

  Measures taken to prevent his escape are insulting and purely symbolic:

  The cage's very existence was an insult. Kandra were not like men - even if he were not imprisoned, TenSoon would not have run or tried to escape. He had come willingly to his fate.
  And yet, they locked him up. [...] It was made of iron plates and hard steel bars with a strong wire mesh stretched across all four faces to keep him from reducing his body to base muscles and wriggling through. It was another insult.

  _{-- Chapter 20}

  But he has already consciously rebelled, at least to some extent, in his impassioned speech to the other kandra. This was his first premeditated act of rebellion, and one which perhaps makes it easier for him to make the decision to rebel even further by escaping 'justice'.

• The most crucial moment is during chapter 20, in his conversation with MeLaan. This is a kandra he respects and cares about, urging him to rise up against the Second Generation and lead the younger generations in a revolution. While he discards the idea and argues against her, he is pleased that his words about Vin had at least convinced her. And, as muru mentioned in a comment, the biggest turning-point of all is when he realises that events outside are moving even faster than he'd thought, and that Vin may need him after all. His knowledge of the world above is a year out of date, and MeLaan updates him:

  "You were right, TenSoon," she said. "Ash cloaks the land in a mantle of black. The mists come during the day, killing both crops and people. Men march to war. Ruin has returned."
  TenSoon closed his eyes. "They will do something," he finally said. "The First Generation."
  "They are old," MeLaan said. "Old, forgetful, impotent."
  TenSoon opened his eyes. "You have changed much."

  _{-- Chapter 20}

  Closing his eyes for several moments shows the strength of his unspoken reaction to MeLaan's news, even though his words are as submissive as ever. His comment about how she's changed may also foreshadow his own change. His thoughts come back to the same issue later:

  He had returned to receive his punishment because that was right. It was more than honour, more than Contract. It was who he was.
  And yet, if the things MeLaan had said were true ...
  Ruin has returned.

  _{-- Chapter 20}

• After this, he does set the events in motion that lead to his escape, by mentioning the dog's body, but he's still unsure. He does so only to give himself a possibility, to leave his options open; he doesn't necessarily plan to escape.

  It had taken him quite some time to appreciate the advantages of those bones.
  He paused.
  But, no. He had not come to bring revolution. He had come to explain, to serve the interests of his people. He would do that by accepting his punishment, as a kandra should.
  And yet ...
  There was a chance. A slim one. He wasn't even certain he wanted to escape, but if there was an opportunity ...

  _{-- Chapter 20}

• He then spends an entire chapter agonising about what to do, whether he should take the 'honourable' course and submit to the Second Generation's sentence or whether it's now his duty to find Vin and help her. I'll try to quote only the most relevant parts:

  Days later, MeLaan's words still pricked TenSoon's conscience. [...] now, strangely, an eternity of imprisonment seemed like the easy way out. [...] by so doing, he would leave MeLaan and the others to be destroyed as their leaders refused to take action. What's more, he would leave Vin without the information she needed. [...]
  The end could be nearing. If it was, then Vin needed to know the truths about the kandra. Their origins, their beliefs. Perhaps she could use the Trust itself. Yet, if he told Vin anything more, it would mean an even greater betrayal of his people. Perhaps a human would have found it ridiculous that he would hesitate now. However, so far, his true sins had been impulsive, and he'd only later rationalised what he'd done. If he fought his way free of prison, it would be different. Wilful and deliberate.
  [...] Escape was not something he preferred to contemplate - it simply wasn't the kandra way. He had broken Contract, and deserved punishment. There was honour in facing the consequences of one's actions.
  Wasn't there?

  _{-- Chapter 24 (emphasis mine)}

  Even when he gets the dog's bones, he's not sure whether he should use them:

  Dared he hope? [...] "What is wrong with the bones I now wear?" TenSoon asked carefully, pulling over the sack, uncertain whether to be excited or ashamed.

  _{-- Chapter 24}

• It's only by the time he actually comes to face his sentence that he's fully made up his mind:

  It had been a wild hope, but he'd manipulated the Second Generation into giving him back the dog's body. [...] The comfortable bones gave him strength. This was the body that Vin had given him. She was the Hero of Ages. He had to believe that.
  Otherwise, he was about to make a very big mistake.

  _{-- Chapter 33}

• The next time we see him, he's back in Luthadel, and this is the first time he seems entirely free of doubt that he did the right thing by escaping:

  I should never have left her, TenSoon thought, feeling a stab of anxiety. My foolish kandra sense of duty. I should have stayed here, and told her what I know, little though it is.
  The world could end because of my foolish honour.

  _{-- Chapter 52}

TL;DR: his change of mindset is slower than you've made it sound, but the biggest turning-point is when he learns the world is ending sooner than he'd expected, Ruin is already working actively to destroy it, and Vin may have need of him.