Here's the real reason why the author of That Time I Got Reincarnated as a Slime was inspired to draw Rimuru as someone cool and strong!

allowing them to witness the true evil of the Kingdom of Falmuth, and then proclaiming himself as a loyal follower of Rimuru Tempest, a monster that wants to ally with humans, worked much more.

In episode 5, after Falmuth's Archbishop Reyheim gives his account of what happened on the battlefield during the intended invasion of Tempest ... of Demon Lord Rimuru and the Evil Dragon ...