This is incorrect. Murderous overrides all diplomatic settings.
However, it still has limitations. You cannot attack if you have already been declared a defender, and sometimes the process of declaring defenders can be a bit zealous.
Furthermore, murderous or not, combat or not, has no effect on whether you can loot another realm. That is entirely determined by diplomatic status.
I can loot the realm in question, but only if I can clear the troops of that region. I can't do that if I can't engage them. So it isn't a matter of diplomatic status.
