Is it fair what happens to bonzo? Any sympathy for him? Does anyone ever deserve this?

In Chapter 12, Ender's fight with Bonzo is very similar to that with Stilson. He does not want to hurt either one, but they both pushed him to act decisively, as he sees it, in order to avoid future attacks. In both cases, he kills his antagonist. This time, however, Ender does not wonder if he is like Peter. He realizes that there really was no choice in the fight with Bonzo, that he either had to hurt Bonzo enough to stop him or be killed by Bonzo. Even so, afterward, Ender is sad again that he has needed to act so violently. Ender might have tried to hurt Bonzo less in order to teach him a lesson without injuring him so severely, but the risks there were great. Perhaps it was more moral for Ender to try a lesser response anyway, and readers may be tempted to think that Ender chose the easier, more decisive path. But we have seen in the fantasy game that Ender's usual motivation is to try to move peacefully before choosing violence as a last resort. The one exception is the snake in the tower, although with Valentine's influence, he returns to his pattern of peaceful interaction wherever possible.