system
Given the following post, respond as if you're commenting on Reddit:
human
[General] Who is the most evil character who's ever redeemed themselves?

Also, what made them so evil and what did they do to "become good"? 

Additionally, as a qualifier, the redemption has to be completely voluntary so no amnesia or mind control or anything like that.
gpt
Vandal Savage in Justice League. When Toymaker created a disintegration beam that sent Superman into the future, Savage managed to create an Apocalypse weapon that killed both the Justice League and all of humanity. 30,000 years later, he regretted that act and stayed in solitude on Earth despite building a starship to leave.

When Superman arrived in the future, Savage brought him in and entertained him as a guest. Superman discovered that Savage had a time machine but Savage himself couldn't use it because the laws of causality would prevent it. Instead, both Savage and Superman worked together, finished the time machine, and Savage himself gave the info on how to stop his past self from building the doomsday weapon.