One who has little faith in human sincerity and goodness

One word substitution is Cynic.

Egoist: a self-centered or selfish person
Fatalist: one who accepts all things and events as inevitable.
Stoic: a person who can endure pain or hardship without showing their feelings or complaining.
Cynic: a person who believes that people are motivated purely by self-interest rather than acting for honourable or unselfish reasons.