Yeah. Szayel did change.
But the sad part is, even if Nemo could have come here and talked to him in these last moments, they would have ended up fighting. Because Szayel is all about pain and hurting people. His pain, his fear, the harms he inflicts on others with glee...
He's Aizen's, through and through, because he lives in Aizen's world, breaths Aizen's way of doing things. He made a friend, yes, but a friend like Gin is to Aizen. Someone who helps him and understands him and sees his greatness and helps him further it...
And someone who knows exactly what kind of monster he revels in being. I looked outside of myself, he said, and he was... God, Szayel was even proud about it. It really was a big step for him. But if Nemo had come to him, explained everything to him, begged him to join her cause and protect the people Aizen was going to sacrifice...
He wouldn't have understood. He couldn't have. He looked outside of himself, and he finally saw someone else, and he didn't think: 'oh, other people matter'. He thought, 'oh, this one is special'.
In another hundred years (or sooner, if the stars aligned, or never, if they did not, but there was no more time and the lines are drawn) atop a mountain of dead and tortured test subjects, maybe, maybe, Szayel could have learned if Nemo carefully led him to be a better person one step at a time. And in another lifetime, she might have done that. But not here, when she has so little time, when she has a faster way to stop him from hurting people. When the time came to draw a line in the sand, Nemo saw that Szayel stood on the other side. It wasn't a surprise (she knew his monstrosity; not 'he is a monster' though there's an argument to be made there, but 'he acts like a monster') and she didn't hesitate. (But she had to drive a knife into his back to do it, metaphorically. And that hurts everyone involved. You can betray someone for a good reason, but it is betrayal still.)