It would be only just if the Palestinians would get their own state after this.
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The current situation is like somebody commits a murder. Then the community rounds up a posse and goes out to kill the murderer. Then kill the murder's family, their neighbors, the residents of the next neighborhood over, raze the neighborhoods and then take all the land for themselves.
Justice means penalizing the guilty parties, not everyone in their geographical/social group. Your definition of Justice is leaky.
Obviously if the nazis and hamas had come out and surrendered we wouldn't have had to.
But sometimes one has to take out evil. Be it nazis in Berlin or Hamas in Gaza.
And it also serves as a lesson. A lesson certain countries might soon have to learn the hard way:
don't vote for evil, and if you did, don't be the ones who line the streets to cheer for them like Germans and Gazans did.