Tyler's final scene...

It's possible Tyler was involved, but also if you look at the Alex scenes after he and Jessica broke up, he is consumed by guilt over the fallout from "the list", and pretty much crying for help every time we see him.

It's a good observation, but it would be odd that he'd select the person who stood up for him as his first victim. You'd think he'd choose Bryce if he really loved Hannah.

Hopefully, we don't get a farcical second season with Tyler crossing people off his Billy Madison list

A few things support Tyler attempting to kill Alex in my mind.
1) Tyler bought the gun.
2) After we hear the shot and before we hear the ambulance we see a boy furiously riding a bike, and it is not Clay.
3) When Tyler is "getting a sweater" he is clearly putting a gun away in that chest.
4) Immediately after "rescuing" Tyler from bullying in the hall Alex basically tells him to "f" off and leave him alone.
5) He is clearly planning some kind of mass retribution; either a school shooting or a series of set up cluster "suicides" for all of the people he holds responsible for Hannah's death and/or his own bullying.

Also, not concrete, but Alex seems like the kind of kid who would have confessed to his perceived wrong doings to his dad once the deposition became a reality.

Yeah, I think it was Tyler. The show wouldn't have gone out of the way of placing him buying the gun for no reason.

He wanted to shut Alex up since he didn't seem to care about telling the truth.

The problem is no one will remember this by the time a hypothetical season 2 rolls around. they badly needed to wrap it up on this go-round. He doesn't really have motive for seeking revenge on anybody, save Clay for disseminating the photo of his skinny white boy ass nude.

Yes all those photos in the darkroom are of his future targets. But him shooting Alex was odd, since Alex stood up for Tyler in school (although right after Alex told him to leave him alone) Alex was one of my fav characters. He looks like Kurt Cobain to me.
I thought Tyler has a bunch of guns and ammunition in the trunk in his room,,,like he was going to repeat Columbine. If there is a second season, I hope they nip that storyline in the bud real quick. I wouldnt want to watch an entire season of that.