Referring to this news, https://sg.news.yahoo.com/rody-team-steps-save-abu-000000697.html,
according to Buddhism, beheading of people will only result in oneself being reborn in the HELL REALMS. When it is time for the terrorists to die, it will not be pleasant. And they will remain there for millions of aeons and aeons. Thereafter, reborn as animals to be slaughtered again and again. And thereafter, being reborn as hungry ghosts for another millions of lifetimes. Lots of pain and suffering. They just don't get it now. If they know what is awaiting them, they will not do such terrible acts of evil. It does not matter that they think they are of another religion. Karma does not recognise religion. It is not determined by the Buddha or anyone for that matter. It operates by itself. And that is why, it is fair. The fairest system ever.