The fact that they kill so much and still consider themselves "Good Christians" is one of the best characteristics of the Iscariots.
Scenario: Anderson is in the Iscariot confession room taking confessions and granting forgiveness to sinners. Maxwell, Heinkel and Yumie feel that they need to confess their killing sins. However, not feeling very proud of those sins and because of the fact that Anderson can be over moralistic at times, they decide not to let Anderson know it's them, so they make their voices deeper so that he can't recognise them. Heinkel and Yumie get their confessions, but Maxwell isn't so lucky. In the end Father Renaldo arrives to get a confession (remember folks, it's the silent ones that are the biggest killers XD) and isn't too surprised at what he sees.
Many people might think "WTF!? Why does Anderson care about how many people they killed!? He kills all the time." True, but he has more will than they have. Maxwell doesn't care who happens to die, and Heinkel and Yumie will kill anyone if they're ordered to. Anderson however, isn't like that. He'll disobey an order instead of killing someone he feels shouldn't be killed.