IdiotJed
Glutton for punishment
I think that comparing their reaction to that and Xander's motives are two entirely different entities. The fact of the matter is he saw them making out and knew that if it had gone any further, more of his friends would die. The slayer has a responsibility for the protection of the world, sans warm fuzzy feelings for a vampire. She should be on a pedestal of constant watchfulness especially if her honey is the guy that killed a lot of the class. Was jealousy involved? Of course! Xander's a guy with a jones for a hot chick and he'll use any means to make the other guy look bad, but in this case, it was warranted. It just happened to work in his favor. I'm sure his motive was more for not dying than high school jealousy.