A teacher who unequivocally advocates murdering families should be fired, yes.
In this case, however, that isn't what he was saying. His tweet clearly indicates that he wishes that those who (in his view) perpetuate the cycle of violence could be the ones experiencing the tragic results, rather than those who want to break it. He's not endorsing murder (he's actually quite clearly against it), he simply sees death as an inevitable result. Having accepted that, he would prefer to see it isolated to the group who he sees as responsible for the issue.
It's a perfectly acceptable statement, whether you agree with him or not. The sad fact is that people who lack basic reading comprehension have worked themselves up into a frenzy over nothing, as per usual.