Should "to" be dropped?

No, the 'to' means that Moslems would be offended, but others may argue it doesn't offend Islam, it could be just another interpretation. Removing the 'to' means other non-Moslems accept it's insulting.

Cool.
Thank you Izzy.

Not much difference here, but I would suggest

A movie insulting Islam: that was the purpose of the movie

a movie insulting to Islam: the insult was incidental; there were perhaps other facets of the film. Muslims were insulted, others were not.

I would say that there can be a difference between "insulting" and "insulting to".

A film insulting Islam is a film which objectively explicitly and deliberately sets out to insult Islam, whereas a film which is insulting to Islam is one which whose makers might or might not intend to insult Islam, but which is subjectively perceived to do so by the person or people calling it "insulting".

"John is a pervert" both deliberately insults John and is likely to be perceived by John as insulting to him, whereas he might not perceive "John sleeps with transvestites" as insulting to him. He might think it is fine to do that, or he might think it is shameful.

PARIS (AP) -- France ...
The prime minister said he would block a demonstration by people angry over a movie insulting to Islam as the country plunged into a fierce debate about free speech.

PARIS (AP) -- France ...
The prime minister said he would block a demonstration by people angry over a movie insulting Islam as the country plunged into a fierce debate about free speech.

=======================

An action contemplated but not yet taken.

A: That will be insulting Islam. ???

B: That will be insulting to Islam.

=================

An action taken.

A: That was insulting Islam. ???

B: That was insulting to Islam.

==================

An action contemplated but not yet taken.

A: That will be insulting Catholicism.

B: That will be insulting to Catholicism.

========

An action taken.

A: That was insulting Catholicism.

B: That was insulting to Catholicism.

We know what the writer means to say, but "insulting to Islam" implies a singular entity "Islam" that has the capacity to be insulted. That's preposterous, of course, so we take it to mean "insulting Islam."

Cool replies.
Thank you all.