Why did Muslims kill Theo Van Gogh?

And why do all the radical Muslims quote him and tells other not to do things to end up like him? Why aren't these Muslims arrested with these threats? I bring this up because I was in a bus today and I was talking with a Muslim about Islam, apparently I said something to offend him (it had to do with how Islam treats women) and he told me to watch myself before I end up like Theo Van Gogh