The whole point of the Tazer was to give police officers a non-lethal instrument to use against combative/non-compliant/potentially dangerous people. When has a Tazer ever been called lethal force? LEO's are trained to use Tazers and other non-lethal weapons to subdue people in an attempt to avoid using lethal (IE Shooting the ****er) force.
Also, in order to have a Tazer, you have to get shot by one. At least thats how it is with the SL Sheriffs department. My buddy (Deputy Scott Curtis, I'm not making this up) said that its a really weird sensation, but it wears off quick and then just feels like pins and needles. The video of him getting Tazed was hilarious though!
Would it have been better if they had OC sprayed or maced him? That would have put other people in the library at risk of exposure. People have died from mace and OC, does that make it lethal force? Additionally, and this is a point that has been visited before, how was the kid acting before they tazed him? The video doesn't show and all the news reports I've found simply say that he wasn't complying with the police's requests. Did he throw a punch? Did he try to run? If thats the case, then theres your probable cause.