I don't need to imagine it after playing Atelier Sophie. Generally speaking I play games quite carefully, but then once I made the mistake of thinking a boss slime is still a slime. How tough can it be?
Come on, don't look down on the series so early on. Rimuru is a monster himself now, so what's wrong about associating with other monsters? Think of his personality back when he was still a human: He was a dude willing to help his friends, and even gave his life to save them. Clearly not a person only working for his own personal benefit and otherwise ignoring everybody and their distress.