Based on...
Just a quick question... wasn't this movie based on an older movie (might've been a foreign film)? I can't find anything referencing an older version anyplace... it's possible I'm just thinking of Nikita.
Thanks.
Take your pick. The killer encountering a little girl and being kind to her at the beginning and evading a huge security force looking for him by hiding behind a gas mask to kill the bad guy at the end is taken from This Gun for Hire (1942).
The lone meticulous killer in a spare apartment with one small object of love (e.g. Leon's plant, this killer's small bird) is from Le Samouraï (1967)
The child rescued by a mafia figure on the same floor of an apartment building after his/her family was massacred and later propositioning this adult for sex came from Cassavetes' Gloria in 1980.
http://www.imdb.com/title/tt0110413/board/thread/2406531?p=2&d=21898239#21898239
There's also Tiger Bay (1959), Hayley Mills' pretty amazing film debut. She plays a bratty 12 year old tomboy who harasses and snoops on the neighbors in her tenement building and oversees a murder. When the police come looking for the killer, she steals his gun and ultimately goes on the lam with him.
I don't think Leon steals anything from this movie, but the seeds of the trope are certainly there. The film is definitely worth a look if you don't know it. Hayley Mills is astounding.
http://www.imdb.com/title/tt0053361/?ref_=fn_tt_tt_1