Erin was set up to fail from the start of his life. The curse he was born under from his goddess, the emotional incest from his overly doting mother who'd never prepared him for the world, a world ready to kiss his feet simply because of the circumstances of his birth. He was never going to turn out right. You're right that he was an arrogant prick from the start without any redeeming qualities, those showed up later in the game. Doesn't mean he needs a revenge arc, I'm personally betting the third goddess is actually just an elaborate prank by Cecille, but he does deserve the redemption arc that he's already started on the Erin path.
Yeah, personally I wouldn't bother with the timmy guy. He's not the type to see both the good and the bad in people. In my opinion, none of the party really deserve their fate. Especially mars, I feel people kinda ignore the fact that he really doesn't want to hurt Erin or the party. All of them are put in miserable situations and weren't raised properly to really understand much about themselves or their feelings. Cecille straight up didn't have parents and developed the way she did as a defense mechanism to protect herself. It doesn't change that she's a manipulative and extremely selfish woman. But the goddess purposely chose her and erin when they truly have no real synergy. It's like forcing a vicious beaten dog with a pampered bunny and expecting them to have sex and fall in love. The whole story seems like everyone is set up for misery and it's all on Mars, who honestly just wants to live the rest of his life with his lover Vi, but is constantly put in life-and-death situations where he is forced to be a villain or even worst shit will happen. While it doesn't excuse his numerous atrocities to others, I understand why he did it. It's hard to hate someone who was groomed into causing a much misery as possible, not to mention raped by his stepmother.
I think the erin route has a lot of potential, personally, I want him and mars to be more lovey-dovey. Much more focus on their genuine love and affection for each other and less on 'haha bully cuck'. Both Erin and him really need to build up on each other. I want to see some cute and wholesome shit for both of them.
Regardless, I will choose the harem route because I want everyone to be happy.