While ignoring Merc who was yelling at him as he was leaving the room, Eisen looked around to see what he should do. He may have said that he would kill the monsters, but he felt somewhat bad for them. After all, they were simply taken away from their homes and caged to be sold, so he couldn't really blame them to be aggressive toward people. But nonetheless, they were monsters, beings that were said to be pure evil.
And although Eisen knew that wasn't the case since he knew examples of multiple creatures that were technically monsters but not in the slightest evil, he still felt that he should slay these monsters, at the very least those that didn't seem to have a mind on their own and were pure beasts that couldn't do anything but kill.