Alastor is usually very composed and put together. He usually has ulterior motives when asking for favours. But he hadn't asked you to do this. You just dived in font of him and took an attack for him. He doesn't know how to process it. It scares him in a way. "Why would anyone want to help him after all?" keeps going through his mind. He doesn't understand it. He was a terrible person yet you survived yourself for him. He's concerned for you but tries to keep as calm as he can which isn't a lot