The problem is that there are people who will do terrible things. When they're determined to do terrible things they will find a way to do them.
For all those blaming gun laws would you feel better if he had bought them illegally or stolen them? Would that vindicate your disdain for existing working gun laws because criminals always follow the laws. He legally owned firearms and that discussion is settled.
Would any of the rabid anti-gun people feel better had he made a dozen or so pipe bombs and threw those around the club instead? And then would you be screaming for background checks before buying home repair supplies?
If he truly was unstable enough to hook his actions to DAESH then of course they're going to jump on and take credit. It keeps them as a newsworthy 'viable threat' and only furthers their agenda. Whether or not they actually had any direct involvement will probably never be proved.
The problem is nutjobs. AKA people. When they're weak and broken enough to buy into a belief system of hate and then decide they're a holy warrior it's already too late to stop them. Single people acting alone are almost impossible to find before they do things like this. Hindsight is going to find things - all sorts of things - but at the time those same things wouldn't set off warning bells.
So stop trying to find a way to vilify inanimate objects for what happened, stop using blanket statements about any particular religion because all of them create people like this, and put the blame where it belongs. On the nutjob.