It's less retarded and more they have no self-esteem. Look at the MCs in romance stories who are like 'There's no way that person would ever like me' when the chick's throwing her panties in his face and dry-humping his lap screaming 'FUCK ME' in his ear. It's because he doesn't believe he's worthy of the positive attention, so he puts himself down beneath them. Rimuru's done the same here - and has had this fear in his heart ever since he connected with the monsters - that finding out he's only superficially 'one of their kind' would make him a vile traitor using them for his own ends and they would shun him. Fear, anxiety, low self-esteem, these are serious problems for anyone.
Even back when Rimuru is still a human, another human killed him and just stabbed him in a broad daylight, why did he be so naive that no human will be like that in this world, I mean sure morality and all that but now he's a slime FFS
it not fast like WN so stop complain pls....it adapt from LN so of course it more detail, i have read the LN version and so i like this more than the WN version, it have more emotions than an plain story