That's a no true Scotsman fallacy. You can't just discount anyone who doesn't condemn it as not representative of Islam.
I don't know if I buy that these people are merely a fringe group. There are currently 16 Islamic countries where blasphemy can be punished by imprisonment or death. I'm not saying all of them would necessarily endorse beheading for drawing Muhammed, but that's a big chunk of the Muslim world where it would be illegal, and it's not surprising that kind of backdrop can foster extremism.
Bold: that's what many fail to realize. Because yeah fine, even if you prove it to everyone that Islam doesn't literally tell you to behead your teacher because he conducted a freedom of expression class, you can't dismiss the countless phrases & teachings in Islamic literature that normalizes violent behavior against the other, whether the other is an infidel, a gay, a blasphemian, or even a Muslim with of a different sect. That kind of backdrop can definitely foster extremism.