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.
And it will be very painful time for the person when he will find that his coins from his address has been used by another person, so the better and moral option is that we do not have to use those bitcoins which we do not own and we do not have earned with our work, but of someone else.
yeah i think people who intend to steal the wallet though it is not used by someone that but I'm sure people are taking money from the wallet others are evil and immoral. Whatever the reason stealing is wrong and harmful to others. no benefit resulting to others when you stole