Tris's Not-Really-A-Friend Friend
Friends are people who will keep your secrets, laugh with you, and also not dangle you over a potentially fatal fall into a rushing river in an underground chasm. According to that, Al fails the friend test in one crucial moment, when he joins Peter and the other jerks in attacking Tris.
Cuddly Teddy With a Grizzly Streak
Will may have betrayed Tris while under mind control, but Al doesn't really have much of an excuse. In a way, that makes his story is straight up tragic: he betrays Tris because he's frustrated by her. In other words, Al turns out not to be such a good friend when he wants something from her that she can't give.
Through most of the time we know him, Al is described as big and kind. He gets compared to a grizzly bear during his first fight in the arena (9.41). But most of the time in the ring, he refuses to fight as hard as he could. He throws his matches simply because he doesn't want to hurt people, which sounds a lot more like a teddy bear than a grizzly. As Tris recognizes, "Maybe he is too kind for Dauntless" (10.18). Tris may be uncertain about that—it's only "maybe." But we think that's pretty accurate, considering how Al's been forfeiting his fights (14.31).
In almost any other circumstance, Al would be a hero (which is why his suicide is so tragic). Al joined Dauntless because he wanted to protect people (16.12). He might be scared of Eric, but when Eric orders Christine to hang over the chasm, Al is the one who starts chanting support; it may be pretty bland to say, "You can do it" (9.86), but it's also pretty brave to take Christina's side against Eric.
So why does friend Al turn into villain Al? Well, it's important to recognize that he doesn't become a villain. Even when he helps Peter to kidnap Tris, Al isn't a bully. When Peter starts to molest Tris, Al rebels: "'Stop that,' he snaps." (21.139). The fact that Al "snaps" might emphasize that he's not thrilled with this. But even if Al feels mixed feelings about what he's doing, even if he apologizes to Tris later, let's not overlook the fact that in this moment, Al has taken sides with Tris's enemies.
You could argue that this lovable big galoot made this bad decision because he likes Tris and she's rejected him (16.19-25). And that's surely part of it. But we like Four's interpretation: "He wanted you to be the small, quiet girl from Abnegation" (22.29). In other words, Al wanted to be friends with someone who needed him. Or in other other words, a true friend accepts you as you are, and Al can't do that for Tris. He wants her to fit into a category that she won't, and when she defies his expectations, it leads to tragedy.