I thought once different about it.
But a victory of IS, FSA & Nusra or Assad would bring big problems for the future with it.
Hezbollah and Iran will use Syria for their plans without any limitation. This should be not allowed.
Syria would develop in a concentration area for Sunni terrorists of all kind if IS wins or FSA Nusra. They will forget the huminitarian help Israel grants them in Southern Syria instantly and start attacks.
Neither side can be controlled by any power. Some people thought they could controll Hitler and helped him to win politically. It was the worst idea ever.