A: When a reliable sourceunambiguously describes a character (and not just their actions) as a villain (or villainous), evil, a bad guy or a baddie. Note that a "bad boy" is not the same as a villain and a description of villainous acts that one interprets as the character being a villain is original research (see relevant discussions: 1, 2).
Q: A character that used to be a villain is now no longer a villain. Should they be removed from this list?
A: No. Characters can change all the time, and as long as reliable sources describe them as villainous at some point in their history, they remain listed. The character is still villainous in those episodes (see relevant discussion: 1)
