I would think that anyone advocating for or cheering the death of civilians would be valid reason for removal. Criticizing Israeli policy, being supportive of Palestinians in general, or contradicting Netanyahu's talking points: not a valid reason for removal.
Those aren't great either, but a mitigating factor is that those things happened a long time ago in conflicts that have already ended and few of the people involved are even alive anymore. It's not like the United States is seriously considering firebombing Desden or Tokyo or using nuclear weapons against Japan again in 2025. (At least, not yet as far as we know.) Having opinions about history is different than having opinions about an ongoing war.
There's also a difference between people who say that those attacks were a least-bad option to win the war from a utilitarian ethics point of view versus people (presumably a minority) who simply see no down-side at all to deliberately killing large numbers of German or Japanese civilians.
