Earlier quoted context omitted.
Buddhism has a fairly explicit prohibition against killing or even working for companies in the business (i.e. gunsmith.) This goes even as far as not executing serial killers. So this is unprecedented, which is part of why many are so shocked.
Almost all religions have prohibitions against killing; almost all adherents ignore religious rules whenever they would require acting against their immediate self-interest. Buddhists are no exception. Sad as it may be, such atrocities are not unprecedented and have accompanied Buddhism throughout its history; just as they have accompanied any other faith. If you are interested, the book "Buddhist Warfare" collects a…
Buddhism, from it's source, is different in that its first espouser categorically denounced violence against all living things, which makes any Buddhist warfare a shocking example of human ills. The Buddha even goes as far as to denounce just retribution.