The bugger queen's thoughts mentioned "We did not mean to murder, and when we understood, we never came again." Similar to how Ender "finished" his future fights with Stilson and Bonzo, the IF finished the buggers. Despite this, Ender's decision to use the ansible against the Formic homeworld is never justified. The Third had enough of the adult's games and lack of respect and took his frustration out on the "simulation." Even he didn't think his actions were justified; "I didn't want to kill them all. I didn't want to kill anybody!" Ender's impulsiveness and resentment brewed from mistreatment by the IF resulted in the lost of ten billion lives that can never be given back.
Q4. Ender never redeemed himself from his violent actions against humans, but he did (or is going to) find atonement with the buggers. The xenocide of the buggers did not make up for the deaths of Stilson or Bonzo. For example, most believe that Ender's redemption had been fulfilled after the deaths of ten billion "hostile" aliens, but the buggers had not once again been near Earth for years counting; they had stopped attacking and humans were no longer in danger. What Ender had "saved" them from was