During an active military conflict, killing isn't applying the death penalty just for disagreeing with someone, it's typically the only remotely viable way to stop them committing whatever act they're in the middle of. When that act is genocide, killing them is almost universally the only moral action as anything else, even if it still stopped them, would take longer and in doing so let more genocide happen.

If they've been stopped by other means, e.g. economic sanctions make genocide too expensive to continue, then many genocidal acts carry the death penalty under international law. That's ethically dubious, but it's far from the biggest problem with anti-genocide law given that it's blatently not actually preventing genocide. If the username were just apply international law to zionists, then it'd still be promoting killing people.

It's also misleading to reduce zionists to certain type of people, as it conjoures up ideas based on inherent identity that are obviously bad to persecute before more directly comparable types of people like murder enthusiasts actively committing murders and refusing to stop despite pressure to. No reasonable person would say armed police were unjustified in shooting someone who was stabbing someone else after they'd been already told to drop their weapon. Not all zionists are actively killing people, but they are all calling for it to be allowed to continue, otherwise they inherently wouldn't be zionists.