The ICJ decides on the facts and procurable evidence on the basis of international law, that is why Israel is showing up to present a counter claim. Hamas and Israel are the two parties, and Israel cannot wait to face their accusers.
Last time Israel did not show up when the ICJ ruled that the West Bank Security barrier was illegal in their opinion, and it was said if Israel had showed up to defend its position, the decision might have been different. Genocide my asp!
The only possible problem I see is that Article 4 ,1. The Members of the Court shall be elected by the General Assembly. — This is done by secret ballet, and 57 members of the UN General Assembly are Muslim States, the largest voting bloc at the UN General Assembly.