Everything is written on the terminals onboard the Prydwin, how Maxson became Elder, etc. They are not bad people, they are one of the few that actually help the people of the commonwealth. Look up things like "The Glow", The Master, and The Calculator on the Wikki (or play F1, F2 and Tactics) to see why they have the thoughts about Synths, Ghouls and Supermutants that they do (and no they don't kill regular ghouls, they just don't like them because they dug up their ancestors and messed up the graves in a sacred place called 'The Glow'). Actions speak louder than words in this game, look at the actions of a group over their words, and you get the truth.

Maxson orders that all technology is to be traded for food and medicine and forbids taking it by force. He also forbids killing innocents. Maxson agrees to let Senior Scribe Neriah develop a better version of Rad-Away to help the people of the commonwealth. There are vertibirds and troops everywhere clearing out Raider, Supermutant and Feral Ghoul nests. Not saying they are saints but they do have redeeming qualities people seem to overlook because of first impressions.

Same thing happens with The Institute, and the Railroad (this is explained in Far Harbor DLC, won't ruin it for those who haven't played) a lot of people don't see what lies beneath the clean and polite exterior presented, even though the terminals tell of kidnappings, horrific experiments on those people and the killing of those victims. And they don't care at all about anyone above ground, they are waiting for us to die out before going up there and starting over. Father and the Courser companion X6-88 mention that.