-Support
You've put yourself into more trouble than him, you didn't take care of the DD well enough, realistically a drug dealer would be equipped with some kind of weapon, and the fact that they were behind you, and could have easily attacked you from behind to prevent you from robbing customers would require you to have to deal with them good enough, and you didn't, breaking...
6.3 NPCs- When role-playing within the vicinity of NPCs (Non-Player Characters, for example, the NPC that resides within Fredy’s Bakery, the City Hall, etc.), NPCs must be treated as regular players, where possible; for example, if a player has used just committed a serious crime in the suburbs within the vicinity of the storage NPC they must threaten and/or take the NPC into consideration. Another specific example is if a player is involved in a bank robbery and is visiting the hospital to receive treatment from the NPC the aforementioned player should ensure his and the NPC’s lives are not in immediate danger.
Now onto your apparent problem here, you gunpoint them both, one of them (and like a total badass might I say) saw an opportunity and took it, he dove behind cover to give himself enough time to pull out a MUCH more powerful firearm, and then pop you in the head. He had enough time and cover to do this realistically, and that's why no rules were broken by him.
