It appears Jamie killed Katie after being influenced by corners of the internet that convinced him he was too "ugly" to receive affection from a woman, as he lets out in his conversation with the therapist in Episode 3. When Katie turned down his advances and bullied him with an emoji that implied he was an incel, he did something tragic.

Dive into the shocking ending of Netflix's No. 1 crime series, "Adolescence." Why did Jamie kill Katie, and what happened to him and his family in Episode 4?

They are eventually told that, while messages between Jamie and Katie appeared to be friendly, the emojis actually suggested that Katie was mocking Jamie for being an incel—meaning involuntary celibate, which is connected to the rise of misogynistic culture, particularly among young men.