Everything Lauren-Clear-Monica-Ipsum writes is true. Lennie did indeed kill accidentally just because of his nature, and without understanding what he did or why. George does want to spare Lennie from the pain of a trial, a hanging, or a lynching. Yes, he could have led the lynch mob away.
But George realizes an important thing. Even if he rescues Lennie now, there will one day be another woman and another lynch mob. He won't be able to protect Lennie for ever. Not only will Lennie still get to suffer, but there will be another dead woman in the future. It is better to have Lennie die now, at peace and with his friend, than painfully in the future that must come, and with another woman needlessly dead.
There are likely to be other factors. In good literature, as in life, decisions like this are rarely for one reason. Lennie does deserve to die. He did really kill the woman. Lennie's death will free George from the intolerable burden of caring for him. It will mean George won't have to lie to cover for his friend. And any future killings by Lennie would be on George's conscience, since he could have prevented them. Deciding which factors weigh most with George is part of the enjoyment of the book.