the easiest answer is that they are aliens and they dont trust us but if you think about it you might say "hey... thats a difficult question" also they look alot like humans
its better to make a peace treaty with something that looks like us than to fight it
ps: i think the species scientific name will be sine misericordia interfectores which translates to "merciless killers" in latin to english it makes sense cuz they even kill civillians... i think or they just hate the civil protection and mistaken the player as a soldier of the civil protection which they are not