a little help

ok so I saw the I guess h anime school days and the end was confusing.first it looked like they got along but no they wanna kill? Why, what happened, who started it? Sianzi? or the other girl ? I think it's sianz's cuz she killed him. And the other wanted him alive? why did the kid say thank goodness? Plz help me .I almost cried and I'm a guy ? Why such shifty end dammit!