Angels and Demons

Imagine if a nation of psychopaths were to appear on some land. These people are anti-social to the extreme and are out to destroy the rest of the human race. They have no higher goal and they seem to be no convincing them against their evil ways. There may be ways of convincing them to renounce those ways but no one has yet to find a way to convince them. They have developed sophisticated war technologies and are about to wage war on the rest of Mankind. Mankind seems to have adequate ground to kill many of them in self-defense or to incapacitate or incarcerate the rest. They would have no justified moral grounds to complain against such treatment.

Now imagine a nation of angels, of perfectly moral beings that were to appear on some land. They have also developed very sophisticated war technologies but their reasoning for doing so is to protect themselves from attack from the rest of Mankind. What principled, non hypocritical grounds would we have against a proposal by them to kill and/or capture and incapacitate us?