"I'm sorry But I now know why I cheated on you. Because you were so rude."
The ex's message is a textbook example of a 'narcissistic injury' response. By sending flowers, they were testing if you were still under their control; when you rejected the gift and set a boundary, they immediately pivoted to verbal abuse to regain the upper hand. The claim that they cheated because you were 'rude' is a classic DARVO (Deny, Attack, and Reverse Victim and Offender) tactic designed to make you doubt your own reality. They are projecting their own lack of healing onto you to avoid facing their own failures. Do not defend yourself against their accusations, as any response—even a defensive one—is the 'supply' they are looking for.
The ex is attempting to seize power through verbal abuse and gaslighting, but the user holds the actual power by refusing to engage in the debate. The user's threat of legal action is the only thing currently keeping the ex at bay.
This is a post-breakup interaction where the user is attempting to establish a hard boundary (no contact, threat of legal action) against an ex-partner who is attempting to re-engage via 'thoughtful' gestures (flowers) and subsequent verbal abuse when rejected.