system
Given the following post, respond as if you're commenting on Reddit:
human
[Batman]. Why doesn't Mr. Freeze go by Doctor?
gpt
"Mister Freeze" was a name given to him by newspaper reporters and internet journalists who didn't know his true identity. They just saw cold-based crimes, a dude with a cold gun, and used a dumb but appropriate name to get the point across in headlines. In most universes, it's not even revealed to the public that Victor Fries is his real name until long after Batman locks him up for the first time. But by that point, more people are familiar with "Mister Freeze" than any other label, so future news stories keep running with it. Also a few might be vindictive, especially if Fries hit something they care about, and prefer to not use his real name just out of spite. 

Victor Fries refers to himself as a doctor all the time, even if no one else does. Although after spending years as an outlaw, he tends to stop caring about the formalities of society, and disconnects himself from everything except his obsessive mission. So he stops caring what people call him because they literally do not matter to him.