Sid's body was made by Cain after Sid consume some sort of demon, in doing so his mind became crowded kind of like DID? This makes Sid's body unstable and that's why he says he can easily tear apart. He is mentally unstable. Satan makes a kind of pact with Sid to quiet the voices in Sid's head in doing so Sid has to obey Satan's orders but Sid is of course devoted to Cain and plans on stabbing Satan in the back but with all of these new inputs messing his plans including falling for Jinyoung in his own twisted way Sid is now even more confused than ever. He can't stand to see Jinyoung smile at others so he wants to end him which is messed up yes. Sid doesn't completely understand the emotions he has for Jinyoung. Moving on from that Sid isn't completely evil his plans so far have not been to completely destroy others. He has tasted the peace of a sound mind and that is the sort of blackmailing power Satan has over Sid, but Sid wants to be loyal to cain and be able to be by Cain's side. There is so much more to Sid but I'll leave you guys to find that out!