I came up with a line when taking part in a gangster RP set in London and I wanted to get that real cockney sarcastic tone but with a real sense of danger in it like in Guy Ritchie films etc

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"Listen, there are two kinds of enforcer. Bees and wasps. Wasps will buzz around do what they like and if they want to, they'll sting you. Just because they can. You, my delightfully ignorant friend, are a wasp. The second type of authority is a Bee. Now a Bee will buzz around doing what it should. Working for it's queen, it's colony. A Bee wont sting you unless you provoke it, but if it feels threatened and it decides it's time to show his assailant what it can do its sting is nearly ten times worse than that of a wasp. You don't remember a wasp sting, but you'll remember when you anger a Bee. I, as you have probably guessed through my gloriously constructed metaphor, am a Bee. And if you provoke me, I'll make sure you fucking remember it."

Other honorable mentions - 

God of some sort - "I'm about to do something that sickens me. I shall give you a choice."

Staff Sergeant - "The more you sweat in practice the less you bleed in war."

Space faring Wrench monkey - "The Ark was built by an amateur whilst the titanic was by professionals."