Is there sin in one's feeling distressed by dealing with people and believing that most of them are hypocrites, which leads him to prefer formal relationships and distrust others, especially after a painful past experience?

A Muslim must think well of people because it is an act of worship, and thinking ill of them is forbidden by the words of the Almighty: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." What is forbidden is to confirm an ill suspicion about a Muslim without any doubt, not merely what crosses one's mind without settling there.

As for the saying "people are ruined," it is blameworthy if it is said to belittle people and prefer oneself over them. However, if it is due to sadness over a deficiency in religious practice, then there is no harm in it. This pessimistic view negatively affects one's self, hinders one from goodness and good interactions with people, and may lead to isolation. One must get rid of this view and not assume people are inherently bad due to a single bad experience.