This is a bit late to be posting about Trust, but here goes.
One of the questions on the viewing guide for Trust was, "Is Matthew an existentialist?" My answer, in a word: no. Existentialism demands the acceptance of one's life. Meursault certainly does this. Whether refusing an offer to move to Paris or contemplating his inevitable death, Meursault rarely thinks what could have been, and so is content. Matthew, however, thinks entirely differently. Where Meursault is happy, Matthew is depressed. He carries a hand grenade with him at all times, in case his despair ever becomes too much. The grenade is not an idle threat, either, as he proves at the end of the film. In addition, Matthew subscribes to common morality. He despises the gas station attendent who attempts to rape Maria. He thinks Maria's mother is immoral and sadistic for treating her daughter so poorly. Overall, Matthew absolutely is not an existentialist.