I don't think Merle abused Daryl. I think they were both abused by their father. Daryl looked up to his older brother as a protector, but Merle left Daryl alone with their father at the first chance he got. Daryl still didn't hate Merle, though. So I don't think Daryl was taking his anger out on Merle's walker. I think Daryl was just mad that Merle was dead.