>>283690868
You just can't comprehend the concept that there's plenty of schizo people even in reality who won't perceive themselves as harming someone by killing them.
This is really the culmination of Denji's insanity allowing him to circumvent traditional logic that'd make a Devil unkillable. Which is why Kishibe calls him the "most suited person for the job".
Despite people seeing P1 as a "happy ending", it's really meant to show that he's left as a deeply disturbed individual, which is why the P2 events happen subsequently.