You may be correct, as far as their fears of unbelievers infecting them or other islamists.
However, I think to a great degree they are trying to be true followers of their religion. Their holy book tells them to kill unbelievers, so ISIS and "radical" islamists are following it to the letter.
I think another factor is the mental disposition of people involved. They all learn violence and killing from an early age, and some of them delight in their barbarity. Basically those who would be considered a part of the criminal element in our culture are revered in theirs.
The people prone to violent acts can direct them toward infidels or other people deemed undesirable and be looked upon as a great person in fighting for Islam.
Sorry for the quick edit. I'm running late so I have to go.
I agree and think that your observations are aligned with mine.
I do think that both the prevailing liberal and conservative assumptions are very defective, though.