You ever have one of those weeks that just gets worse and worse? On Monday you miss the bus and get fired, Tuesday your girlfriend breaks up with you, Wednesday your parents disown you, and so on. Bad things usually come in threes, but if you think about it, 12,000 is a divisible of three, so technically that can mean 12,000 bad things.
That's what happens to the main character in this movie, but with much higher stakes. On Monday he kills a cab driver on accident, and then it snowballs from there. His girlfriend is the next day, followed by his brother on Wednesday. Then he just keeps killing. The good thing is he's a butcher, so hiding all that killing isn't too hard as long as his customers don't mind lean meat.