I'm going to make an obese man pay me cash to take up part of my seat on a long flight
Why would you make him pay you cash?
It's not my fault that obesity is becoming a major health threat these days.
Travel can be annoying but trying to get money out of people because they are overweight and in your space is not good. Others will think that you are petty.
I only decided to do it because he was eating chips.
That isn't an excuse to be rude. Dealing with people on flights is something we all have to do so that doesn't give you an excuse to be rude.