I actually think this season was a thoughtful evolution. Luther has always been presented as a cop that stops just short of rogue. He bent a few small rules at first. But he has continued to evolve, as the world around him has gone to shit.
This season, events pushed him over the edge to breaking the law. AT one point, Luther takes Alice to the police station, and she mocks him about doing the right thing (turned out he was doing online research, but still). So Luther's character is still there.
Later, Luther positioned it nicely himself at the end, talking to Halladay about how waiting for backup and a warrant could mean that women who are currently alive would then be dead. He wants to break all the rules, regardless of cost, because that is the quickest way to take action, and maybe save lives.
This season, Luther firmly and completely crossed to the other side of the line, because when you are dealing with murderers (George) and psychos (Alice) all day, your perspective changes. Plus, Luther always had this potential flaw in his make up. That's what made him such an interesting character from the pilot.
The interesting thing this season was that, when Luther did cross the line at the end, it made no difference. All the people in the house were already dead. AND Halladay was shot and killed by Alice outside. So he fully threw away his last vestiges of integrity, *and it didn't matter."
Now, he has been arrested. Will it stick? Guess we won't know unless there is more. If there is no more, then the show's ending message, and our character's arc, is sure depressing.