International law is one area where whataboutism is a perfectly valid and accepted line of argument. Unlike domestic law, there is no enforcement mechanism for stuff like war crimes and most laws aren't clearly written down. If a country like Russia or the US executes some prisoners of war, the war police isn't going to show up to write them a ticket.

International law only exists where countries agree it exists. Part of that agreement is treaties and the like, but most of it is what International lawyers call "state practice". Countries can say that a law exists all they want, but if they don't follow the law in their actions then the law doesn't exist.

Therefore, "America, you consistently fail to follow this law so I'm entitled to act as if it doesn't exist" is a totally reasonable argument as far as International law is concerned.