Any stealing act for anything to anyone is still illegal wether it is online pr not.All stealing avt is considered a crime as long as you dont have permissions of the real owner
The real owner? Who decides that if it's possible for me to generate the same address? Just because I generated it after shouldn't mean it's illegal. It's not like exploiting an online wallet to steal private keys and drain the accounts.
I feel like you guys are just guessing to be honest, I want to know if it's legal or illegal to take bitcoins out of an address you just generated.

No stealing bitcoins is not illegal because they dont know who owns the bitcoins because they are untraceable and that means that you are free to will to steal other their bitcoins