Impunity of members of UN security council
If a UN resolution is vetoed by at least one of the members of the UN security council, the said resolution is thrown in the trash. That means that if a war crime is serving the interests of at least one member of the UN security council, its full impunity is de facto guaranteed. Even worse: some of the members are notably war-thirsty and 2 of them aren't even democratic. And there is no way to change this way of functioning because it also can be vetoed by the said members. How many crimes have been made possible because of that?