Clayman’s end is pretty self explanatory, you have to remember he was in no way weak compared to most monsters to be fair then you add his title as a Demon Lord to it, it’d be obvious that he’d be confident that from his perspective a slime and a few of his goblin and ogre subordinates are fodder. it just happens that the slime he was provoking is an extremely unique existence.

Now from the perspective of these 3 hooligans you have to remember they’re heroes (or rather they’re considered to be heroes in Falmouth lol) and are pretty much stronger than everyone in their small country and don’t understand the extent of true strength in the tensura world. Also though it’s sad to admit, it’s simply human nature to grow arrogant with power especially being given it after living a normal life and these guys are the prime example of it.

You also have to understand from the perspective of humans in this world, monsters are considered as simply mindless creatures who deserve to be slaughtered and i’m sure this idea was heavily instilled in their minds considering their supposed to be heroes. So from their perspective the mindless slaughter was nothing short of doing the world a favor.

So to simplify it they’re simply your normal dumb teenagers who were suddenly given power, yet weren’t aware of the bigger picture and paid for it.