Probably because he's lived the last 3 years with demi-humans who actually appreciated him, unlike humans, and he came across a bunch of dicks who were about to kill an innocent lizardman because they didn't want to get in trouble for abusing him?
Thanks for the chapter. @Countrymage yes, it is.
When he killed demi-humans he felt conflictuelle
Now when he kill humans he fell nothing.
He will make a great demon king
(It is not a spoiler when it is in the title)