I don't think the characters were stupid at all
I know a lot of people have said the characters were stupid for not leaving when Eden and David started acting strange, but I think their decision to stay makes perfect sense. First, most people would never expect their friends or family members to try and kill them - even if that person is acting a little off. I think this was a big factor in why they chose to stay - because they had a history will Eden and David, and by all accounts, that history was great prior to them acting strange.
Think about it: if someone goes on a first date and that date is acting weird, most people probably won't call them back. They don't know anything about the person and don't have a prior good history which would make them at least try and figure out what's wrong. If someone has been in a very happy relationship for years, and suddenly their spouse starts acting strange, most people don't break up at the first sign of trouble. They try to work things out, see if their suspicions may be unfounded, etc. If that doesn't work, then they break up.
Most of the other people had a long history with Eden and David, and probably never in a million years suspected a friend would try to kill them - even if that friend started acting strange. So that's why they stayed.
Also, the context in which Eden and David started acting strange has to be taken into account. Even Ben said "I'm all for whatever you gotta do to get by." Most people picked up on Eden and Ben acting strange, but also gave them the benefit of the doubt because Eden lost a son, and her strange behavior was her way of dealing with it - even if that meant living in denial.
Just my thoughts.