Other mistake: During the pub fight scene, the guy who got hit with the glass cup (Wound on his forehead), pulls a six shooter out of his pants, but he fires eight bullets at Harry's umbrella.
Other mistake: In the "Uncut" version of the film, after the "Dinner Party" at Valentine's place, when Valentine is at the door saying goodbye to Harry, he says "Just give me a couple of days to think over your proposal." His lisp has disappeared on the word "days." (00:57:00)
Other mistake: When the Kingsmen are looking on-screen at the dossier of the princess, the attached photo of her is just a screenshot from the earlier scene where she is at the dining table with Valentine and the Swedish prime minister, just before she is imprisoned. Her photo was not taken at that point, and couldn't have been attached to her private file.
Other mistake: In the final scenes where Eggy goes to have a fight at the pub with his mother's boyfriend they are listening to Slave to Love by Bryan Ferry. The counter says 962 whilst the tape machine is playing (electronic), but the counter does not change.
Answer: Details aren't given, but it seems that Dean is involved in local crime along with the gang that hangs around with him. Even if Harry is bluffing, and doesn't have specific proof, it is clear that he is abusive and he has the recording of him threatening Eggsy.