There's a lot of miserable people out there. No matter what they have that causes this, they are often toxic to others. I find it's best to just avoid them. They usually have a very negative aura and like you said, complain about everything and/or criticize everyone.
In some instances I don't think a disorder is necessary. Some people are just lazy / entitled and prefer to complain and criticize others over improving their own lives. That way the focus isn't on them but on everyone else (while still gaining attention and sympathy and making it about them).
But I know for sure that narcissists are extremely negative and complain about everything and criticize everyone, because of their enormous sense of entitlement, delusional views and expectations of others, relentless projections of their own insecurities, and so on. They are also permanent victims in their minds.