The lyrics describe a whirlwind romance that seemed perfect at first, but soon revealed its dark side. The narrator sings about how she knew there were red flags from the start, but ignored them because of the intense feelings she had for this person.
Throughout the song, Swift uses metaphorical language and imagery to convey the emotional turmoil of the relationship. She compares her love to a "crimson curse" and a "drug" that she was addicted to, despite knowing it was harmful.
The chorus emphasizes the narrator's realization that she was in over her head and should have seen the trouble coming: "I knew you were trouble when you walked in, so shame on me now."
The bridge of the song builds up to a climax as the narrator expresses her frustration and anger, wishing she could go back and undo her decision to get involved with this person. She realizes that she let herself be blinded by her feelings and paid the price for it.
The final chorus repeats the main message of the song, reinforcing the idea that the narrator knew her love was troubled but still allowed herself to get caught up in it.
Overall, "I Knew You Were Trouble" serves as a cautionary tale about the dangers of ignoring red flags and giving into intense emotions in a relationship, leading to eventual heartbreak and regret.