Yes, Bashar Assad is behind the war crimes against the citizens of his country, because he has looked the other way while these atrocities have gone on in Syria. He is the leader of the Ba'ath party. That party has been the party in charge while these terrible things have happened. He has done nothing to stop it.
Assad is the head of his country and the absolute dictator of the land - his entire family has run the country for the better part of a century, and he is an absolute ruler. Assad could end everything, but instead he and his family have a personal hand in the crimes.
There is abundant evidence that Assad is behind war crimes in his own country. He has used chemical and conventional weapons against civilians, and he refuses to end the civil war. He is clearly a megalomaniac who should be removed from power. The West should intervene to remove him immediately.
The evidence is so overwhelming it is just impossible to make a case suggesting otherwise. Assad is a butcher, a murderer and a tyrant that would rather burn his country to the ground than relinquish control of it. As we all stand idly by doing nothing, that ultimately will be what he accomplishes.
There is no doubt in my mind that the war crimes that continue to ravage the people of Syria are directly happening because Assad has ordered them. People don't make decisions like that spontaneously. That is corruption that comes from the highest level. The buck stops with him, and he is a war criminal.