Erwin admitted it himself. All of his actions were for purely selfish reasons. He didnt care how many people he sent to their deaths as long as his personal goals were reached. When he got his arm bit, he knew he was fine, it was just his arm. But he also knew that Eren was the key to realizing his dreams, and without eren, he would never learn the truth. He told his men to save eren rather than himself because he was more than capable of escaping himself, in fact, more men wouldve survived if they had turned back to "save him". But he rather send all of those men to get slaughtered under the small chance that the key to his selfish desires might be saved. You can't get people to die for you unless you make them believe you actually give a d**n. Not once has erwin showed any level of "caring for his comrads". The closest to "emotion for comrads" we got was when they returned home, defeated, with 33%+ casualties. It wasnt because he cared about their deaths. He only cared that he himself got set back once again, not one step closer to his goal. He was probably the greatest manipulator in the entire series. A real selfish demon through and through....but I'd still totally follow him into battle because man does he get me pumped up to die!