http://youtube.com/watch?v=VP_M8s0GFEc
The video doesn't show much of anything, it's more audio than video.
First off, people are allowed to be in the library that aren't students up til 11pm from what I understand, after which only stundents are allowed. This kid didn't have his ID, and was asked to leave. Eventually he did, but it took him long enough that rent-a-cops were arriving when he did.
Rather than escort him out, they decided to stop him on the way out. Grabbing his arm and not letting go when asked is a big no-no. If you're at that point, you should be ready to put hand cuffs on the guy.
There were at least 4 cops shown in the video, more than enough to simply cuff and drag away an unarmed person. They decided to use a taser instead.
The video didn't start until after the first taser firing, but from the way he was acting at the start of the video, he might have been screaming and fighting. At one point he was screaming like a moron and it sounded like he was flailing his body around. This might have been when they put hand cuffs on him, because he appeared to have his hand behind his back when he was tased the second time.
This is when students starting asking for badge numbers, which the cops were right to simply ignore at this point.
Now they've got him handcuffed and tased him twice, he's not all that coherent shouting like a mad man about abuse of power. There's at least 3 cops there, but rather than drag him away handcuffed, they repeatedly ask him to stand up.
Two cops drag him away after a few minutes, and a third cop apparently stays behind to calm down the crowd. He was asked for his badge number after the kid had been taken out of the room, and responded by threatening to use a taser on this other kid. You don't work crowd control by threatening violence when you haven't been threatened with violence yourself.
Tasers have become abused as much as mace by police, it has the same effect as punching someone in the nose without the lasting damage that can be used as a case for brutality.
The cops didn't handle the situation properly, when they tried to stop and question the kid and saw him not cooperating, they should have been ready to cuff him or let him leave. They spent too much time trying to get the kid to do what they wanted when it was apparent that he wouldn't.