It just hit Dan that what Alex is doing isn’t funny but dangerous. It was episode 174 where Dan has his realization that Alex has always been super racist and how that affects the things Alex says and does.
When the show started they covered current broadcasts. Soon after they started doing regular time travel episodes where they would put Alex in ‘time out’ and cover a past episode. Episode 174 is one of those episodes. It’s the juxtaposition between the two times that put it all in focus for Dan. If you are looking for best episodes to go back and listen to, 174 is near the top and really addresses your questions.