Letting a known predator go free (molester, rapist, serial killer) for money.
Killing an innocent (especially a child)
Using an innocent to acomplish a run (selling a child to a molester)
Undeniable urges (IS a serial arsonist/rapist)... but trying to reform
Used to make especially graphic, sick BTLs
Involved in the murder of a large group of innocents (cleared a village to make way for a corp mining project)
Killed best friend as a child
Didn't stop someone from doing something nasty (Best friend from beating his wife to death, etc)
Used to sell women to bunraku parlours
I can think of alot more, (those are tame ones) but basically it should be somethign horrible that the character used to do/did that he feels bad about doing now. Even if he didn't mind doing it then.