"You cannot do wrong and feel good. It's impossible" means that every bad action you do, you'll never feel good. It is true because we know that we chose the wrong so, we feel wrong,. So trying to feel that you did something good wouldn't work. It's our feelings. Imagine you badly injured someone on purpose. You just wanted to get that person with prank, which badly goes wrong. In that prank you were suppose to drop ice and water from a huge bucket from the roof of the house onto that person. Now that person is injured. You have two decision, help that person and pay for the consequences, or leave him to possibly let that person die. You decide to leave that person in the ground. That action would simply haunt you for the rest of your life. So what ever you do, by doing something wrong, you'll never feel good.