Yes when u say stealing then yes its illegal.And bitcoin is a currency its also a money so stealing it surely illegal but what you are talking about is dont think gonna happen.The chance to that happen is very very less.
Bitcoin is money but not a legal money so that is not illegal, I hope you know guys what are the difference of not right and illegal. Maybe it is not right to steal but you cannot say that it is illegal because bitcoin is an independent currency and the government does not recognize it.