There are three Unforgivable Curses in the wizarding world. Most others will earn you time in Azkaban, but as long as no one died, you’ll eventually be released. These three, however, earn you a lifetime sentence in Azkaban: the Cruciatus Curse, the Imperius Curse, and the Killing Curse. The Killing Curse may also earn you a Dementor’s Kiss, a fate worse than death.
Harry Potter is the only known survivor of the Killing Curse, and not just once, but twice. Voldemort used this curse on Harry when he was a baby, but because of his mother’s protection it bounced harmlessly off of Harry and hit Voldemort instead, unintentionally creating a Horcrux with Harry as its vessel. Because of the Horcruxes Voldemort had already knowingly created, instead of the rebounded curse killing him it turned him into nothing more than a wraith, a bodiless entity who couldn’t perform magic or any other basic function unless he was possessing someone.
Voldemort attempted to use the Killing Curse on Harry a few more times without success, usually having the curse deflected harmlessly by someone else, until that night in the Forbidden Forest. Because of the Horcrux inside Harry, when the curse was used on him that time, it was the Horcrux that died, not Harry. Because of his willingness to die he was only knocked unconscious, temporarily comatose while he and Dumbledore had their conversation at King’s Cross. Once Voldemort realized Harry was still alive, he attempted to use the Killing Curse yet again (you’d think he’d have learned by now). This time Harry deflected it with Expelliarmus, his “signature move,” as Remus Lupin once called it. Only this time, instead of forming a golden cage like it had in the graveyard, and probably because he was using the Elder Wand instead of his holly and phoenix feather wand, Harry was able to push the curse back at Voldemort, and this time he had no Horcruxes to save him, so his own Killing Curse killed him.
Harry Potter also seems to be the only person not a Death Eater to get away with using not one, but two Unforgivables in front of witnesses. Granted, he also used them in front of close friends, so I don’t think he has to worry about them turning him in anytime soon. He used the Imperius Curse on the goblin at Gringotts so they could break into Bellatrix Lestrange’s vault, and then when one of the Carrow siblings insulted Minerva McGonagall, he used the Cruciatus Curse on him, wanting to inflict as much pain as possible for daring to disrespect someone he held in such high regard. Carrow was actually lucky Harry would never use the Killing Curse on someone, or he might not have walked out of that room. You don’t mess with Harry’s family, and Minerva McGonagall is high on the list of people Harry claims as family, whether he outright says so or not.