How many times does Ender attack people in the novel Ender's Game?

One before Battle School, right after he got his monitor taken out, Stilson.. ends up dying.

One in the Space Shuttle, Bernard, breaks his left arm.

And then two times in Battle School.. Bonzo tries to fight gang up on Ender in the bathroom and he gets beat up pretty bad, ends up dying.

There's also before Bonzo, when Commanders sent their soldiers to Ender's evening practice, to try and stop him, they tried to fight but because of the null gravity, everything went wrong. They lost and Ender got some good punches in.. drew some blood.

So I guess all in all, four times. (And Ender never knows that he killed anybody)