Don't know if he was actually connected with ISIS, it seems he was a lone gunman and this wasn't a co-ordinated attack, but the connections to ultra conservative Islam is evident; rational and moderate Muslims wouldn't dare to do stuff like this because of the highly contradictory nature of Islamic doctrine; they are basically able to separate right from wrong from their religious texts.
We do however, need to notice that this is exclusively a problem with Islam. As bad as the WBC is, with regards to Christianity currently, they are not actively going around killing innocent citizens for what they believe in; rather they are just preaching regressive rhetoric. Salafi and Wahhabi Islam are actively sponsored by states such as Saudi Arabia, where Sharia Law is legally binding, and the state and the faith are enforced together in dictatorial fashion. This is part of the reason I hated Ted Cruz so much, the man actually wanted to enforce a theocracy onto the citizens of America. Islam as an ideology is extremely contradictory, and extremely aggressive with its stances; the interpretation is left open to the follower. Like I said, most Muslims are able to separate the good from the retrograde and integrate into western civilisation, but because these regressive and aggressive ideologies are still sponsored by states in the Middle East, it doesn't at all surprise that the system produces people like this. I don't necessarily think it's fair to scrutinise the subjects from a collective point of view of an oppressive system, as opposed to the system itself; especially in one as authoritarian as Salafi and Wahhabi.
Yeah, I'm not talking about religion though, I'm talking about mental illness. I don't think religion was at the root of the problem here: it's an angry man who allegedly beat his wife and became enraged after his son saw two men kissing. He may have found inspiration to carry out this attack in religious texts or from religious teachings, but I don't think a level-headed American citizen turns to that level of radical Islam.