It doesn't have to make sense to us. The hundreds of thousands of his own people he has murdered - estimates are over 450,000 dead civilians - over the years he has been in power never made any sense to us either but he did it nonetheless. If nothing else, he is acting now the way he has in the past.
As noted above, he seems to need little reason to kill his own. Four years ago he fired gas-carrying rockets into a suburb of Damascus, killing an estimated 1,300 civilians. The only reason ever put forward was speculation that the area was primarily Sunni; Assad is a Shiite. Remember, being the wrong sort of Muslim in the wrong place at the wrong time has been a capital offense for over 1,400 years!