The semantics of the teaching method and tools used in the classroom seem to be largely irrelevant to me. The real ‘problem’ for the killer and his ilk was that the teacher/school/state dared, by introducing the very concept of freedom of expression, to question the primacy of their religious beliefs.
The use of the cartoons may have been a tipping point of sorts, but if you’re the sort of nutjob who’s prepared to decapitate a school teacher, the subject itself is going to be an issue, not just the subject matter.