Dear Karen
Daredevil saved you in the first episode of the Netflix world, and you appreciated him before you knew who he was.
Sincerely
Stop your disapproval.
Karen has always been supportive of the concept of Daredevil: someone who fights for justice and does what the cops and legal system sometimes cant. This hasn't changed. At the diner, Karen said "I really think that Daredevil may have made the city a safer place." Her issue is with Matt himself, and there are two factors involved: the events of season 2, and Matts own attitude toward Daredevil at the time when Karen finds out about it.
Karens conversation with Matt in the diner at the beginning of The Defenders indicates that she didnt know he was Daredevil before he told her his secret, although she probably subconsciously knew. Still, she watched Matt's life fall apart in season 2. As much as Karen supports Daredevil, the fact that she now knows Daredevil is Matt Murdock, a man who she cares deeply about, means her investment in him is now personal. She saw firsthand the negative effects of Matt's lifestyle, with his neglecting work in Frank Castle's trial, his random disappearing acts, his fights with Foggy, etc. Now, being Daredevil does not always have a BAD impact on Matts life, but for most of
Daredevil, that had been the case, as far as Karen can see. She cares enough about Matt that, in spite of all of the good work Daredevil has done, and as much as she advocates for Daredevil, she also doesnt want Matt to suffer.
Then there's Matt's own view on his activities after Season 2. With Elektra dead, he decided that he wanted to stop being Daredevil, and he made this clear to Foggy and Karen. He saw his vigilante career as pointless and destructive, so he set himself the goal of channeling all of his efforts into his legal career. Foggy and Karen saw Matt's seemingly desperate desire to stop doing this destructive thing he feels compelled to do, and thus became invested in helping him.
But its more complicated than that (and coming from Matt Murdock, why wouldn't it?), because despite his short-term disgust, Matt actually
wants to be Daredevil, but he doesnt make this clear to his friends because he has a hard time breaking old habits of secrecy and lying to them. That's why Foggy and Karen act like Matt's a drug addict who is trying to sober up when they realize he's done Daredevil stuff: because thats the impression theyve received of the situation from Matt. Remember that Foggy and Karen are not privy to everything the audience is aware of; they're only aware of what Matt tells them. So when Karen learns that Matt has well resumed being Daredevil, she's upset because Matt is once again doing this destructive thing hed confidently told her just days ago he didnt want to do anymore.
Karen is additionally upset that Matt is being so irresponsible about his secret identity, since she knows that bad things will happen to her, Matt and Foggy if Fisk finds out. And she is also upset that Matt has put her in danger again, which is also understandable.