You did the investigation yourself or you watched the news and believed whatever you heard? There are lots of versions of his motives, depending on who you’re listening to. I’m sure some far-right media outlet has “evidence” that Biden hired him.
If he just wanted to kill people, he would have taken more shots. Nothing in his past, that has been released at this point, shows he wanted to kill people for no reason. This was intentional, and seeing as he is a political figure, having this be politically motivated makes the most sense.