Initially planning on taking advantage of their naive companions, Leonard Snart (Captain Cold) soon began to be influenced by the other Legends and started behaving in a more heroic fashion that he ever had before, in Legends of Tomorrow season 1.
At the end of the first season, in a truly heroic moment, Snart sacrificed himself to save everyone else.
When Snart was first introduced in the first season of The Flash, he was a pretty hardcore bad guy.
After all, how can you call yourself heroes if one of your membership killed his family in a fire?