Some fraction signing up so they can gun someone down, sure, those people have bad motives and should be found and excluded.
But being willing to say "I'd kill to protect X if there is no other option" is not that.
Someone who becomes a surgeon so that he can kill a patient now and then and get away with it obviously has something wrong. Someone who becomes a surgeon and now and then causes a patient to die unintentionally (even if they intended to do the surgery, knowing it could result in death) is not the same.
But being willing to say "I'd kill to protect X if there is no other option" is not that.
Someone who becomes a surgeon so that he can kill a patient now and then and get away with it obviously has something wrong. Someone who becomes a surgeon and now and then causes a patient to die unintentionally (even if they intended to do the surgery, knowing it could result in death) is not the same.