Because this is the first time murdering a bunch of hamas members wasn't par for the course historically. Now it's Palestinians not 'Hamas' for the Palestinians, but one-in-the same for the US and Israeli governments. The other side of this is that they are a terrorist organization and need to be exterminated. Hamas says the same thing about the israeli's.

Also, war crimes as a subject is always brought on by the winners, just like the telling of history.

The world is a brutal place. Hamas has proved they are not a military match for israel and the war will continue until israel has complete control of the region. That's how one sided conflicts have happened historically.