Yes! Of course stealing bitcoin is illegal. Still, if you find a forgotten wallet/lost wallet, it'll not be illegal. Because, owner of those bitcoin might have forgot about that.
disagree you are wrong because stealing of bitcoin is illegal in any situation ,so you can not say legal if one person steal bitcoin from other .
yeah it is illegal and i think it is a really great thing because then there are less thieves who want to steal all of my money i hope i will never get my coins stolen