Why would a teenager steal food from a baby?

i have a teenage cousin who was at my house and i was making my 2 year old lunch , i asked the teen if he wanted something and was told no i already ate. i made my baby lunch and the teenager ate it literally she was sitting their eating and he was standing their taking handfuls off of her plate ramming her food down his throat! this is not the first time either it's like anytime she has food he steals it! and when i say something he goes sorry yet does it again and again and then gets mad when you say not to, he also steals besides food and has a lying problem, in a way he tries to manipulate people is this normal teen age behavior? or does this kid need help ? what could i do to help?