Stealing is illegal, yes. However, since he overlapped the address, the former owner of that address wouldn't be able to access those coins anymore and since there's no way to know who that former owner is, wouldn't it be a shame to not spend those coins and let them be in there?
Think about it.
Something very bad thoughts ", However these steps can be done when we do require funds for urgent needs. But if later we find the address of the owner or owners of such addresses to contact you to ask for the return of funds that there then you are obligated to replace