I really think "Bad Little Boy" is a deep episode because Marceline was the one telling the story, so she was projecting how she thought her personality was onto Marshall Lee's character. We all know how Marceline sees herself as a bad person and continously calls herself (1/?)
A "monster". That's why Marshall Lee objectively seemed more evil. He continously bullied Fionna throughout the episode and manipulated her until he made her believe he died which caused her to cry. Although, he did end up showing remorse and he seemed to be apologetic. I (2/?)
think Marceline said the story that way because of what happened between her and Princess Bubblegum a long time ago, she sang a song (Woke Up) to Princess Bubblegum which led to their break up and she didn't show any remorse right afterwards. Marceline portraying Marshall (3/4)
Lee like that is her wishing that she could be more remorseful and not emotionally closed off when she does something she doesn't mean to, like the break up with Princess Bubblegum. (4/4)