"Good" wizards can use Avada Kedavra just as the Death Eaters do. Avada Kedavra is not a curse or spell that has to do with anyone's internal "goodness," as is, conversely, the Patronus Charm. Mad-Eye Moody teaches Harry's year about the Unforgivable Curses in Goblet of Fire:
‘Avada Kedavra’s a curse that needs a powerful bit of magic behind it – you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a
nose-bleed. But that doesn’t matter. I’m not here to teach you how to do it.'
Goblet of Fire - page 192 - Bloomsbury - chapter 14, The Unforgivable Curses
Moody doesn't say that the curse needs a powerful bit of dark magic behind it, but rather just magic.
The reason Voldemort doesn't use Avada Kedavra to eradicate the "good" wizards is that, with training and the internal will to perform the curse, any witch or wizard can perform Avada Kedavra.
Case in point: Snape killed Dumbledore using Avada Kedavra, and Snape was firmly on the good guys' side. While we don't know for sure, it's possible Dumbledore may have killed his own sister Arianna during his boyhood duel with Grindelwald, and it's further possible the curse he used to do it may have been Avada Kedavra.
‘The argument became a fight. Grindelwald lost control. That which I had always sensed in him, though I pretended not to, now sprang into terrible being. And Ariana ... after all my mother’s care and caution ... lay dead upon the floor.’
'You see, I never knew which of us, in that last, horrific fight, had actually cast the curse that killed my sister. You may call me cowardly: you would be right. Harry, I
dreaded beyond all things the knowledge that it had been I who brought about her death, not merely through my arrogance and stupidity, but that I actually struck the blow that snuffed out her life.'
Deathly Hallows - pages 574-575 - Bloomsbury - chapter 35, King's Cross