that's why they should torture/kill criminal then they will think twice befor breaking laws
You don't know that. As of now, they ((un)fortunately) killed their hostages.
If it gets like you said, don't expect hostages to die that easily, and if now they went on a theatre, next time they may go to a school of you torture them. KARMA of a psychopath is painful.
And thus don't expect Syria to survive any longer, which would cause more and more innocent people. Who knows, the boy that was found dead in coastline, everyone felt bad,
War isn't that easy as you say.