There's an old bar on the outskirts of Old Gotham that fills every night with the faceless scum of Gotham's underbelly. They move as a mass towards the dirty, understaffed bar, drinking themselves into oblivion. Sitting round the corner from Arkham Asylum, it attracts a surprising number of ex-inmates, who tend to end up off their face and in the employ of a Gotham Kingpin.
This is where the Joker is headed next.
Inside, the air is thick with smoke. No one looks up when the Joker enters. He stands at the door for a long while; just watching. No one says anything. Rats aren't welcome here.
There's one man who stands out tonight. He's standing at the bar, a beer in one hand and a wad of bills in the other, telling anyone near enough how lucky he is. "Look," he slurs, shaking the bills at a man drowning himself in his drink. "Maroni. 'e's a good guy." He puts down his beer and makes a gun with his spare hand. "One lil hit, and tha's it. He gives me a pile of cash ta keep my mouth shut." He realises what he just said. "Oh, don't tell nobody I told ya."
The Joker reaches into his pocket and pulls out the top card. Glancing at it, he smiles and walks towards the inebriated man. It takes the man a few minutes to realise there's someone behind him. He turns around and stands dumbly in front of the Joker, mouth rapidly opening and closing as he tries to make sense of what's happening. The Joker pulls the top card back out of his pocket and slides it into the man's hand before turning around and walking out the back door.
He turns when he hears the heavy door opening. The man is standing there, holding out the card. "Didja want somethin'?" He mumbles, offering him the card. "Cause I'm real good when it comes ta jobs and stuff."
"Oh no. No, no, no, no, no," the Joker whispers, fingering the knife in his pocket. He gestures for the man to move away from the door, which he does in an instant. "I want you..." The Joker grabs the back of the man's neck and pulls him closer; impaling him on the knife he holds in his other hand. He moves his hand from the man's neck to his mouth, covering it as he pushes him backwards until he slams into the brick wall beside the door, the Joker's weight pinning him in place. "To keep quiet."
The man gurgles and blood runs from beneath the Joker's glove. He doesn't try to say anything as the Joker pulls the blade up through his torso, stopping at his collarbones. There are wet splats on the concrete below as the blood begins to flow in earnest. The Joker smiles and steps back, removing his hand from the man's mouth and his knife from the man's neck.
He watches silently as the man stands for a moment, swaying slightly on unsteady legs. As the man begins to slide down the brick wall, he turns away, walking slowly down the dark alleyway, his footsteps resonating off the buildings surrounding him.
The police would find the body in the morning while chasing down a drug dealer. They'll assume it's just one more example of Gotham's gang violence; until they see the Three of Diamonds, still clutched tightly in the dead man's hand.