No, it's not
You WANT it to be ISIS, because that's a lot easier to process than a old white guy snapped and gunned down a bunch of people. ISIS claims responsibility for any massive attack, so I'm not sure what you mean by it's rare... it's not. A terrorist organization would love nothing more than for people to think they're the brainchild behind every attack.
I don't want it to be this or that. I'm constantly reading and watching the news, gathering the facts as we know them and making my best educated guess on what I think is most likely. It may or may not be ISIS, I don't know and we all don't know until all of the evidence comes out and we know for sure. I think it's safe to say many people want to know what the motive was.
I say that it seems ISIS related because of several key points:
A) The shooter may in fact had help.
B) They found explosives in his car.
C) This was a well planned out attack.
D) And when ISIS makes a claim not just once, but twice it is rare. I base that on what the terrorist experts say.
Are we to believe this was just a one off without any help or guidance from a group? This guy just hated country music? He even cased another venue before choosing the Mandalay Bay.
Now I'm not going to sit here and say, yeah, this is definitely ISIS. But so far it looks that way. It just has that felling to it. It could be nothing more than a case of mental health.