The thing for me, is that yes, watching some of the on-camera response shows some glaring incompetence, but I just can't get to the point of agreeing it explains everything; it's just too much of a leap.
The best I can arrive at, is LIHOP (let it happen on purpose).
Some big media (MSNBC, CNN and key political figures), have been stoking for 8 years, language and phrasing, where they were clearly hoping and praying some deranged nutjob would take to heart and take a shot at Trump; it is very clear many of them were praying their rhetoric would elicit this exact response from one of their more rabid followers. They know anyone that buys their drivel is probably stupid and may be willing to try, eventually. Like Tucker mentioned awhile back, I figured it was a guaranteed eventuality that some dipshit, lunatic, left-winger would go overboard, and here we are, success in their minds.
What I can't get past, in this "just incompetence," theory is that the most obvious, closest sniper position was left wide open, after being perused by SWAT with a ladder...you really think a professional security SS team would all just go up there with their ladder, take a look at the very easy sniper vantage, at very near zeroing-range that any moderate novice could probably hit a paper plate 9 out of 10 shots, and say, "Nah, this doesn't need to be secured, let's just leave the ladder here and forget about it?"
Never mind the other info coming out where they may have gotten eyes on him way beforehand, using a rangefinder and such, or members of the crowd yelling about a dude with a rifle, and the rest of the ridiculousness, I can't get to "simple incompetence" with that one fact; it's just too much to believe that SS, counter-sniper professionals and planners would let that slide or just somehow overlook the most obvious vantage, so close to an obvious target (given the decade of stoking rhetoric) and go even further as to leave implements to make it even easier (like the ladder).
I just can't. I won't go so far as to say they hired a hitter, as this kid was surely not a guaranteed shot and wasn't super-skilled, but I'm leaning toward, "someone was hoping a lunatic would take the bait and take a shot, someday, and when that day comes, we'll 'accidentally' make it slightly easier and see what happens." Because we also know, no matter how bad the fuckup looks, nobody will actually pay for any supposed 'incompetence' that will surely be shown, because, well, it's the Fed baby, nobody pays!