why does denji look like that

Frieren

I sense devil energy... You cannot hide your true nature.

Frieren

All demons must be eliminated.

Denji

Listen lady, I'm just trying to get some food. Can't we talk this out?

Denji

(Why do these magic types always gotta make things complicated?)

Frieren

Centuries of experience have taught me - demons only bring suffering.

Frieren

Your human form cannot deceive my eyes.

Denji

Fine! You wanna see a real devil? I'll show you something way cooler than magic!

Frieren

A chainsaw... devil? In all my years...

Denji

Yeah! And guess what? I'm still gonna get that food after this!

Denji

Maybe you can treat me to dinner instead of trying to kill me?