what is the meaning of this idiom

To really understand what each of these is trying to convey, one would need to know the context in which it was used. "Tearing the life from their bodies". To 'tear' something from something is a brutal and violent action, so this shows that people are being killed violently and brutally. "Snapping life shut". To snap a box or container shut is a very decisive and final action. To transfer the meaning to 'life' suggests that someone's life is ended with a quick, decisive action and that that life is now closed and finished, not to be opened again. "Shepherd of evil". A shepherd is someone who cares for and directs a flock of sheep (not an image one normally associates with evil). Jesus is known as 'The Good Shepherd' and this idiom is using that association to state, almost, a contradiction. It might describe someone or something controlling and directing evil with calmness or it might be used to show that the evil forces are numerous, like a flock of sheep, and that a single person or thing is in charge of them. "Swallow in hot flames". 'To swallow' is usually used of eating or drinking but it is also used of inanimate objects (the sea, a storm, a fog or a fire) when they consume or hide something/somebody completely. It's often used with the word 'up', as in "The ship was swallowed up by the whirlpool. Only a few broken bits of wood were left floating on the surface." Flames are usually hot, of course, but to refer to 'hot flames' emphasises the heat and burning qualities. "Strength was deserting him". 'To desert' is 'to leave'. A desert is an area from which all life has left (not always quite true, of course). Strength is thought of as a positive energy. If it deserts or leaves one, one becomes weaker, so this is another way of saying "He was becoming weaker".