Listening to the FBI Director press conference right now. In my opinion, Omar Mateen was not an "ISIS sympathizer" or a "radicalized Islamist" so much as he was a violent fucktard looking for an excuse to do what he did.
He was investigated by FBI multiple times. At various times, including on Saturday night, he has expressed allegiance to ISIS and also to violent Islamic groups that are sworn enemies of ISIS. He expressed allegiance to the Boston Bombers and to the Florida suicide bomber, none of whom were ISIS.
He also stated (while he was still married) that he wished the police would raid his house while his wife and child were there so he could have a reason to do something like this.
This guy was raised as a Muslim - there is no question about that - but he was too ignorant to even know the difference between the various radical Islamic groups. He just wanted to kill people and needed a "cause" (a/k/a an excuse).
Easy access to guns were the problem here... again. Not his ideology.