Just as some heroes never die, so too do some villains never go away. In the case of Mr. Freeze, his condition made him essentially immortal, or at least, the part he could save. Mainly his head. In the future of Neo-Gotham, he was literally a man living every day waiting to die. Until a young scientist who happened to work for Derek Powers decided to step in and try to save him.
She gave him a new body, a normal one, and he did his best to atone for the mistakes he made. But, when the body began to degrade, said scientist and Powers decided he was expendable, but his body wasn't. Thus, Mr. Freeze returned, more vicious than ever. Ironically though, he was the one who truly brought Blight to the forefront of villainy, and died in his attempt to stop him. May he rest in peace.