"I don't see why not," Percy replied, a tad more confidence in his voice than he really felt. There had been extenuating circumstances that had allowed his mother to overpower Bellatrix Lestrange at the Battle of Hogwarts. She had just lost one of her sons. Bellatrix was trying to kill Ginny. The combination of those two things had allowed his mother the hate that had been required to properly cast a killing curse. There was a deep, sincere doubt in him that most of the "good" guys could actually manage that sort of hate without a catalyst to provide it.
Even if he was sure -- in a way that made him ill -- that he could.
"We out number them by quite a bit from what I could see," Percy said. "They might not have any sort of morals or boundaries where we are restricted by our own consciences, there is a point that will be reached eventually where...what needs to be done will be done."
Percy speak for someone's going to kill them eventually.