According to UN law the one that matters is the security council. But of course, the ICC has already change the rules of genocide twice and still hasn't managed to convict Israel.

That could be an indication that it's just not reasonable. Wait, sorry, what am I saying, CHANGE THE RULES AGAIN! We just KNOW the Jew did it!

UN law isn't really a thing. I see this misconception a lot, that people think the UN is some kind of international law. The reality is that the UN is a debating forum, subordinate to everything else that exists. Countries used to bomb each other a lot. The hope with the UN is that, instead of countries bombing each other, sometimes they will merely yell at each other, agree on who would be able to outbomb the other, not actually bomb anyone, and then lives will be saved. Actual violence is replaced by words and consensus. The Security Council permanent members are countries that you cannot possibly hope to outbomb, because they have nukes (well, the list is somewhat outdated), which is why they have veto power. A veto is the verbal replacement for launching a nuke. You (as a leader of a country) are free to defy a veto - the UN won't come after you, since it's not the law. But you should expect the vetoing country to nuke you or something equivalent, for real. You can also expect no other country to help you - they basically already agreed to that, by agreeing to the rule that lets nuke countries issue vetos.

The UN is 200 different things. From human rights agreements to regulating what network clocks synchronize against and how much a letter from Sydney to San Francisco costs.

Can you also explain what the League of Nations is ... and why the UN was created ? (or should I say, why the League of Nations was renamed)