Warning: contains unmarked spoilers for the entirety of Mockingjay.
Let's consider Gale's role in the plot of Mockingjay
, analysing his character in depth.
Gale equated with District 13
Gale is a wholehearted supporter of District 13 and the rebellion: he's a soldier serving Coin, and more than willing to kill for his cause. In this he contrasts with the more pacifist Peeta - it's not clear how much of Peeta's televised call for a ceasefire is genuine and how much he was forced to say by the Capitol, but in the vote for a 76th Hunger Games he's the first (and most vehement) to vote no. This contrast between Gale and Peeta unifies the romantic subplot with the main plot of the war: for Katniss, choosing Gale would equate to choosing anger and hatred of the Capitol, while choosing Peeta equates to choosing hope and recovery from the past. She says as much in the last chapter:
On the night I feel [desire for Peeta], I know this would have happened anyway. That what I need to survive is not Gale's fire, kindled with rage and hatred. I have plenty of fire myself. What I need is the dandelion in the spring. The bright yellow that means rebirth instead of destruction. The promise that life can go on, no matter how bad our losses. That it can be good again. And only Peeta can give me that.
This is why Gale's personality is so important to the plot. Throughout most of the first half of Mockingjay, when Katniss is working firmly with the rebellion, she's doing so under Gale's influence. He's her closest friend in Thirteen, even if she does occasionally lambaste him for his obedience to Coin. When she takes off on her own mission to assassinate Snow, abandoning Coin's orders, it's after Peeta has rejoined her. And her final choice to kill Coin is motivated by the same event - Prim's death - as her choice to abandon Gale.
We conclude that Gale is inextricably tied to District 13 and to hatred of the Capitol.
While Katniss is firmly on the side of District 13, he remains her friend despite his dedication and willingness to kill for the cause of the war. When she comes to see that the District 13 leadership is as bad as the Capitol leadership, then surely by extension the District 13 soldiers are as bad as the Capitol ones? From this point of view, Gale is just like a Peacekeeper: not inherently bad, but committing murders in the service of a bad leader. It is perhaps symbolic that he ends up working in District 2, where many of the Peacekeepers came from.
The differences between Gale and Coin
But perhaps comparing Gale to a Peacekeeper is too harsh. He is Katniss's childhood friend, after all, and certainly has no ill-will towards her. Her break with him is less drastic than her break with Coin. What are the differences?
Well, obviously, Coin is a leader and therefore responsible for a lot more than the more lowly Gale. But there's more to it than that. Consider the death of Prim - the main motivator for Katniss in turning against Coin and Thirteen. She's pretty much certain that Coin was behind it: the leader of the rebellion is ultimately responsible for actions such as the hovercraft bombing the children. The thing with Gale is that Katniss will never know whether or not he was indirectly involved in Prim's death. Even he doesn't know. He might have been, but all that's certain is that he was a willing participant in the organisation that orchestrated her death.
Perhaps the message here is that by joining such an organisation, you might end up being implicated in atrocities without even knowing about it.
I'm tempted to say that this uncertainty about Gale's level of involvement in Prim's death can be symbolically linked to uncertainty about whether he's sadistic or simply ruthless. But in truth, his character is never in question: it's absolutely certain that he wasn't knowingly involved in Prim's death. His motives are pure - to destroy the Capitol and liberate the people of the districts - and it's only his methods that are sometimes dubious.
Sadistic or ruthless?
Thus, I would conclude that Gale is not sadistic, merely ruthless. Both he and Peeta are at times doing the wrong thing for the right reason. When Peeta calls for a ceasefire, he's wrong to do so, but some of his reasoning is valid - as Beetee puts it (after the downfall of Snow), "[w]e have to stop viewing one another as enemies". When Gale advocates extreme measures against the people of District 2 and the Capitol, he's wrong to do so, but he's motivated by his hatred of their leadership and his grief over the fate of his home.
To support this conclusion, let's look at the way he talks about bombing the Nut:
"Is it really so necessary that we take the Nut? Or would it be enough to disable it?" [...] "Think of it as a wild dog den," Gale continues. "You're not going to fight your way in. So you have two choices. Trap the dogs inside or flush them out."
He's being completely practical at this point - the Nut is a problem to be solved, a nut to be cracked, and he's considering it in the most expedient way possible. It's only when people express compassion for the 'enemy' inside that he bursts out emotionally:
But her anger only seems to infuriate him and he yells, "We watched children burn to death and there was nothing we could do!"
I have to close my eyes a minute, as the image rips through me. It has the desired effect. I want everyone in that mountain dead. Am about to say so. But then...I'm also a girl from District 12. Not President Snow. I can't help it. I can't condemn someone to the death he's suggesting. "Gale," I say, taking his arm and trying to speak in a reasonable tone. "The Nut's an old mine. It'd be like causing a massive coal mining accident." Surely the words are enough to make anyone from 12 think twice about the plan.
"But not so quick as the one that killed our fathers," he retorts. "Is that everyone's problem? That our enemies might have a few hours to reflect on the fact that they're dying, instead of just being blown to bits?"
Back in the old days, when we were nothing more than a couple of kids hunting outside of 12, Gale said things like this and worse. But then they were just words. Here, put into practice, they become deeds that can never be reversed.
"You don't know how those District Two people ended up in the Nut," I say. "They may have been coerced. They may be held against their will. Some are our own spies. Will you kill them, too?"
"I would sacrifice a few, yes, to take out the rest of them," he replies. "And if I were a spy in there, I'd say, 'Bring on the avalanches!'"
I know he's telling the truth. That Gale would sacrifice his life in this way for the cause - no one doubts it. Perhaps we'd all do the same if we were the spies and given the choice. I guess I would. But it's a coldhearted decision to make for other people and those who love them.
He doesn't care about his enemies' suffering; he has no compassion for them; but this is the attitude of a ruthless man, not a sadistic one. He doesn't want to kill them just for the sake of killing them; the capture of the Nut is strategically vital in the war, and that's the objective he's trying to achieve.
And about the traps he and Beetee were developing:
This is what they've been doing. Taking the fundamental ideas behind Gale's traps and adapting them into weapons against humans. Bombs mostly. It's less about the mechanics of the traps than the psychology behind them. Booby-trapping an area that provides something essential to survival. A water or food supply. Frightening prey so that a large number flee into a greater destruction. Endangering off-spring in order to draw in the actual desired target, the parent. Luring the victim into what appears to be a safe haven - where death awaits it. At some point, Gale and Beetee left the wilderness behind and focused on more human impulses. Like compassion. A bomb explodes. Time is allowed for people to rush to the aid of the wounded. Then a second, more powerful bomb kills them as well.
"That seems to be crossing some kind of line," I say. "So anything goes?" They both stare at me - Beetee with doubt, Gale with hostility. "I guess there isn't a rule book for what might be unacceptable to do to another human being."
"Sure there is. Beetee and I have been following the same rule book President Snow used when he hijacked Peeta," says Gale.
Again, Gale's emotional response, justifying his actions in war by comparing them to those of the Capitol, is only a response to criticism of his ideas. He doesn't care about the lives of his enemies, but he also doesn't kill them just for the sake of killing them - these are tools of war, to be used in war, motivated by the purpose of war. The fact that they kill so nastily is just part of their usefulness.
TL;DR: like Peeta's pacifism, Gale's belligerence can lead him to wrong decisions but is ultimately caused by good motives. He's a man too dedicated to a cause, but not an evil one.