The look on meryl's face when he saw daryl's back made me think he didnt know it was that bad. he had that, "whoa wait, whaa?" look. he knew his dad was abusive, but he never realized how far he went with his little brother until he saw the scars.
Or because he cant really see his own because they are on his back, that is the first time he really saw what their father actually did. he knew they were beat, but it made them tough, so he was like, "whatever, if i took it he can take it" but seeing the scars, it finally clicked that "holy crap our dad was a sadistic prick! and i left my brother to fend for himself!"