a) Let's suppose I get in my car and drive to the supermarket. There is a small chance I may accidently kill someone in a crash but, really, this is an acceptable risk. I have done nothing wrong.

b) Now let's suppose I have just drunk two bottles of whiskey and get in my car to drive to the supermarket. There is a far higher probability that I may accidently kill someone. This risk is no longer acceptable. I have done wrong.

What's different? In both cases I have the same intention and motivation : to get to the supermarket. In both cases I perform the same action : get in my car and start driving. In both cases there is a possibility of killing someone in an accident.

The only difference between right and wrong in this case is *quantitative* : the actual numbers for the probability.

"crude arithmetic" sometimes matters.