The psychopath acts on their own desires with no regard for others feelings. Other people exist only to him or her as things to manipulate for those ends. They cannot place themselves in anothers position else they would never hurt them. They care very much for themselves. It would be very rare indeed for one to decide the world needs euthanizing and then do it because they are part of the world. Even the suicide bomber does what they do because they believe heaven awaits.
There are other forms of insanity but the violence done in those cases is more ancillary, more a side effect. Perhaps the maitre d' had one of those and could no longer distinguish internal from external but there is still a threshold of empathy to overcome if one believes others to be like ones self before you can stick a knife in someones head. Hatred can do it but I didn't see that in the protagonist though love can turn that way. Even careful reasoning can do it if a loved one is threatened. It's still quite a hump to overcome and the more empathy the higher the hump is.
There are other forms of insanity but the violence done in those cases is more ancillary, more a side effect. Perhaps the maitre d' had one of those and could no longer distinguish internal from external but there is still a threshold of empathy to overcome if one believes others to be like ones self before you can stick a knife in someones head. Hatred can do it but I didn't see that in the protagonist though love can turn that way. Even careful reasoning can do it if a loved one is threatened. It's still quite a hump to overcome and the more empathy the higher the hump is.