If someone is dangerously tailgating me, I will very lightly brake-check them (barely slowing down) because the vast majority of tailgaters seem to be mediocre drivers who aren't thinking deeply about their actions. If they persist, I'll slow down and let them overtake.

When I'm driving, I'm constantly scanning for bad (i.e. unsafe or unpredictable) drivers in all directions. Once spotted, the choice is either to be well past them or well behind them. Usually the choice is easy.