You can see the cartoon here; I'm not going to post it here.
https://twitter.com/...
In any case, the translation:
Title: "If Muhammad were to Return"
It shows a recreation of the ISIS beheading videos, with Muhammad as the prisoner.
He says "I'm the Prophet, a—hole!"
The ISIS representative says "Shut your mouth, infidel!"
All in all, the message is actually fairly correct—ISIS is violating many of the fundamental precepts of Islam (such as not doing harm to other Muslims). That said, they managed to choose an incredibly offensive way to do it.
First off, Muslims believe that Muhammad's face should never be depicted in artwork, movies, etc. Even in historical films that depict the early days of Islam, no actor ever portrays Muhammad on-screen. It becomes even more offensive that they have him swearing at his attacker; it would be like a movie about Jesus showing him using F bombs.
In other words, I can't say I'm surprised that terrorists decided to attack: this cartoon attacked them and, in the minds of most Muslims, was an insult to Muhammad. That is not, of course, to say that anyone deserved to die for this (especially in a country where the poeple have not agreed to abide by Islamic law); I'm merely trying to point out why this is offensive to Muslims.