No, can't empathize, can't excuse, still can't like Five after reading his POV.
So then, my question is: What's the purpose of this book?
Unlike Six's Legacy where we love her more or Nine's Legacy, where we accept the reasons behind his cockiness and learn to like him, this one does nothing for Five.
Just like Four and Nine, he had teenage angst and rebellious thoughts against his Cepan, but unlike them, at the loss of his Cepan, his sorrow was not enough to change him for the better.
He wallows in self-pity that he's got it worse than the other Gardes, his Cepan is worse than theirs, and that they have gotten together to work and fight together without him. Therefore he turned. But the incidents in this book took place between the deaths of Number Two and Number Three, in other words before we were introduced to Number Four in the original first book. At that point in time, a lot of the Gardes haven't even developed their legacies, much less get together without him.
No, I still don't like Five and I can't even be bothered if he were to do a Severus Snape somewhere in the convolutions of the plot. I like Eight too much to forgive Five.