Ok don't think so hard. When a guardian is "killed" let's say in crucible. They are technically still alive waiting to be revived. If they don't revive themselves in time they will die for good (if I'm correct). A guardian can be killed immediately if what's killing them is dark enough let's say crota or the darkness itself. Different Part: Thanonauts are warlocks that will purposely kill themselves and almost risk dying for good to see a vision of the darkness, the void (where the traveler and darkness come from) or something else.