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.