Mr. Freeze lost all of his reasons to avoid a life of crime during the events of Gotham's multitude of villains in the Dark Knight's absence. One of these villains is the chilling Mr. Freeze, a criminal with a surprisingly sympathetic backstory. Mr. Freeze had cryogenically frozen his wife to preserve her life while he searched for a way to cure her of a terminal illness, but during his efforts he accidentally lowered his body temperature to sub-zero levels, requiring him to stay in his specialized suit to survive. Now he pursues crime as he sees it as the only viable way to acquire the resources he needs to cure both himself and his wife of their potentially fatal illnesses and finally reunite with his love.
Before the events of Gotham Knights, Freeze had struck a deal with Batman. Batman would fund Freeze's research into curing himself and his wife, and in return Freeze would retire from his life of crime. In time Freeze did find a way to cure his wife and revive her, however she rejected him upon learning of the horrible things he had done in pursuit of his research. With his primary motivation of being together with his wife gone at this point, Freeze has lost interest in curing himself of his condition. Now Freeze is different, no longer caring about Batman's deal for funding his research into a cure, and focusing more on research into how to make the entire world feel his icy grip.
Gotham Knights' Mr. Freeze Was Once Sympathetic, But No Longer
Mr. Freeze is different in Gotham Knights compared to previous iterations of the character. He has often been seen as one of Batman's most sympathetic villains, as his entire motive for turning to crime was desperation to find a cure to save his dying wife. He has committed an astounding number of villainous acts in this pursuit, however, and in the end he is a villain because he seems to have no problem with destroying the lives of thousands of people in order to achieve his ends. Since his wife has rejected him, however, he has realized that his villainy is the only thing he has left in his life that brings him any sort of meaning.
The perfect example of this is a minor Harley Quinn who's a dedicated villain in games despite leaning more into the role of anti-hero in recent Batman media.
Mr. Freeze returns to his life of crime as a result of losing the resources Batman granted him to keep him out of crime. However, it can be argued that Freeze's moral com is also beginning to deteriorate, and he has become more interested in villainy for its own sake rather than his original goal of curing himself to be with his wife. Freeze is often seen as a sympathetic character in Batman's gallery of villains, but in Gotham Knights his cold heart towards the citizens of Gotham is on full display.
