It's all on topic since the basic issue is freedom of speech vs. respect for religious differences.
To address this original question directly however, the answer is simple:
1) Death threats should be prosecuted, and should not be tolerated.
2) Drawing pictures of mohammed, both in the original south park and this follow up business should be treated like any hate group literature- legally tolerated to preserve freedom of speech, but shameful and counterproductive.
I would argue that while death threats clearly cross the boundaries of freedom of speech, drawing these pictures violates the spirit of freedom of speech because it's only purpose is to encroach on someone else's life, liberty, and the pursuit of happiness, which is exactly what freedom of speech is intended to protect.