The terrorist weren't upset about the cartoons, they were trying to goad westerners into attacking Muslims. That's the theory, anyway. Should that happen "moderate" Muslims would be more willingly join or at least tolerate the terrorist wing of Islam.
All in all, not a bad theory. If they are really interested in the long game then it makes sense. It probably wasn't part of the Hebdo terrorists' thinking, but they are heavily influenced by the radical Imams who in turn may be influenced by some long-game thinking Islamists.
Result: terrorists swell their ranks for a civil war they long to provoke non-Muslims into starting.
Gavrilo Princip tried it, and he succeeded. On the other hand, John Wilkes Booth thought he could rekindle the Civil War and failed miserably. More recently, Charles Manson thought he could start a race war by murdering Hollywood elites. And he, too, failed. So this type strategy is a long shot, and perhaps it tells us how desperate al Qaeda is for headlines.