After the downfall of Voldemort, what happened to the Death Eaters?
They committed many crimes, including mass murder and torture. Nothing in the books made mention of their punishments. What happened?
In Pottermore, The Malfoy Family gives a hint about what happened in the aftermath of the war:
Abraxas’s son, Lucius, achieved notoriety as one of Lord Voldemort's Death Eaters, though he successfully evaded prison after both Lord Voldemort's attempted coups. On the first occasion, he claimed to have been acting under the Imperius Curse (though many claimed he called in favours from high-placed Ministry officials); on the second occasion, he provided evidence against fellow Death Eaters and helped ensure the capture of many of Lord Voldemort's followers who had fled into hiding. His son, Draco, was saved by Harry Potter during the Battle of Hogwarts, and currently resides at the family estate in Wiltshire.
So we have the capture (and presumably imprisonment) of high-ranking Death Eaters, but the Malfoys escaped prison for collusion. This was corroborated by a post-Deathly Hallows Bloomsbury web chat:
Georgina: Did Lucius Malfoy, and all the other escaped Death Eaters, go back to Azkaban?
JKR: No, the Malfoys weaseled their way out of trouble (again) due to the fact that they colluded (albeit out of self-interest) with Harry at the end of the battle.
JK Rowling said in a 2007 interview with NBC that Harry and Ron “utterly revolutionised” the Aurors, and presumably Ministry justice systems. In Draco’s case, I think Harry saw the capacity for remorse, and might have been willing to offer Draco a second chance.
ETA: One thing that’s clear from the top of the astronomy tower is that Draco is being coerced, not acting out of free will. I think the film showed this best (although the book has a very similar passage):
Dumbledore: “Years ago, I knew a boy who made all the wrong choices. Please let me help you.”
Malfoy: “I don’t want your help. Don’t you understand? I have to do this. I have to kill you… or he’s gonna kill me.”
I think this adds to the idea that Harry would want to give Draco a second chance.
I don’t know exactly what form Ministry justice took post-Voldemort. I’m sure Harry would have ensured that execution was off-the-table (in the spirit of Moody and Dumbledore, if nothing else). From the same Bloomsbury chat:
Steph: Will Azkaban still use Dementors?
JKR: No, definitely not. Kingsley would see to that. The use of Dementors was always a mark of the underlying corruption of the Ministry, as Dumbledore constantly maintained.
So we can infer that Azkaban was still used as a prison, with more humane measures than before. That would be where most of the Death Eaters ended up.
And from the same chat again, we learn that it wasn’t just fully paid-up Death Eaters who got this treatment:
Pablo: What is toadface Umbridge doing now?
JKR: Glad to see you like her as much as I do! She was arrested, interrogated and imprisoned for crimes against Muggle-borns.
I would imagine that Harry, Ron and Hermione all pushed hard for this legislation and retribution (and took particular pride when Umbridge was convicted).
In the latest franchise of Harry Potter collection - Harry Potter and The Cursed Child - Draco's son, Scorpius, was a dear and loyal friend to Albus, Harry's son. Draco also helped Harry by giving him a time turner to reach Albus and Scorpius (probably because his son was in danger too) as they were lost in time. As, Draco became loyal to Harry in the last, he and his family were not imprisoned by the Ministry.
After the final battle, the Death Eaters who had survived it were either captured and sentenced, or then fled after the Dark Lord’s death in an attempt to escape that fate.
“He must speak to the bereaved, clasp their hands, witness their tears, receive their thanks, hear the news now creeping in from every quarter, as the morning drew on, that the Imperiused up and down the country had come back to themselves, that Death Eaters were fleeing or else being captured, that the innocent of Azkaban were being released at that very moment, and that Kingsley Shacklebolt had been named temporary Minister for Magic …”
- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)
Many were captured, but some may have escaped more permanently - there’s nothing indicating how many of the Death Eaters were captured and sentenced, or how many escaped.