
My Exorcist theory:
Chris traded Reagan’s soul for Hollywood stardom.
Chris second guessed herself and tried to weasel out of the deal when Satan sent Pazuzu to collect Reagan. This cost Burke his life, as Satan sent a warning not to fuck with terms.
Merrin’s past dealings made him the only one who had a chance to defeat Pazuzu, but he was weak and easily defeated in the rematch.
Karras knew that a soul for a soul was the only way to get Chris out of it, so he sacrificed himself to redeem his loss of faith.
Chris walked away still famous and responsible for the murder of three people. The film’s true villain.