Turkey has bombed the YPG explicitly because they were advancing too successfully upon ISIS, pushing west of the Euphrates. This, in effect, has created a protected zone for ISIS to operate. This makes him unambiguously an ISIS supporter.
YPG has a different agenda and everybody knows this. And albeit costly, ISIS actually gave them an opportunity. They just want to close the northern border of Syria for their liking. Just do not assume they are peaceful (http://www.al-monitor.com/pulse/originals/2015/10/syria-turk...)
Turkey wants a protected zone to repel ISIS from Turkish border and YPG closing the north line. That's about it.
YPG has plenty of open space to fight ISIS if they want to. Do not overestimate YPG. Without US support, they probably would have lost the game long ago.
