aTmLoKi said:
He taught Harrison the code, but didn't live by it himself. Putting the dark passenger down was the only thing that made sense to bring his story to a close, and Harrison being the one to do it gave Dexter (the man and the show) a fulfilling conclusion.
Very well said. This sums up how I felt about the ending.
He had kept the killing at bay for 10 years, but, once it unleashed again, it definitely seemed to go way awry of the code and he just seemed to enjoy the killing.
I thought his reason for killing Matt was weak. He wasn't a serial killer so much as a jerk who was careless in safety leading to other people dying. He didn't plan out killing them nor did he continue. And Dexter was set off by him killing the white deer, which, while a waste, is not in the murder realm.
I rewatched the finale and noticed when the police chief had him get on his knees to arrest him, he was just about to grab a kitchen knife to use to get away and he stopped just as Logan entered. Then, of course, he killed Logan. So, it was up to two, and potentially three, innocent people he killed that were outside of "the code" already in this series.
The fact that Kurt turned out to be an awful serial killer that Dexter eliminated ended up being simply a convenient "good" he was able to do after that dark passenger showed up again and went rogue.