So much information is omitted without "Veldora's Slime Observation Journal" that's at the end of every volume! Beside the banter between Veldora and Ifrit, they both noticed Yuuki strange behavior, like he openly talks about his lack of skills and from Shizu memories Ifrit knows, and Rimuru should also, how for such a person to become a guidmaster he has to be cunning in politics and how he definitely should already know about Shizu fate and who Rimuru is! And Veldora concludes, after seeing Yuuki geekin out about manga, how no lover of manga can be a bad person!
Again, this is no spoiler, since it's a discussion between this and the next chapter! Beside the conversation those two are having are to be taken with a grain(handful) of salt!