Eh, here's the problem...you can't "criticize Islam" without criticizing individual Muslims who believe in the Islam that you are attacking. You CAN criticize radical/extremist Islamic thinking because now you are focused on the actual problem and the actual people who are part of the problem without overgeneralizing it.
It also sets up a false dichotomy/straw man...that the battle is between Islam is perfect and all Muslims are evil. I don't know anyone on "the left" who thinks "Islam is perfect." Heck, aren't a good chunk of us no good heathens who don't believe in any religion? Islam is no more or less perfect/flawed than any of the other religions. It's got plenty of outright dumb or evil things in it from my POV just like Christianity or Judaism...and it also has plenty of beautiful peaceful things in it just like those other great religions.
Here's the real deal: The terrorists would be terrorists if Islam didn't exist. They'd find some other religion, or create a religion, or come up with some other cause to justify murder/rape/pillaging. So no there really isn't a "causal link" between Islam and the terrorists...other than the fact that it is the religion of the area/choice for a group of folks angry about all sorts of things for which they, primarily, blame the West.
