We come across people in our lifetime that we wish we have never met, but we can't control the way life plays out. There will be people in our lives who WE THINK are nice but then later down the line they begin to show their true colors. Once they show their true colors we begin to think in our heads, "wow! I never knew this person had it in them to ever be like this!" The best thing to do is not stoop down to their level obviously but to let them know that the way they treat you is unacceptable, disgusting and absolutely unbearable. Obviously someone who is known to be a completely awful person to others are aware of their actions, they just choose not to come to terms with them for various amounts of reasons. The best ways you can tell someone that they're being an A-Hole are:
1) Stand your ground!
Don't let someone walk all over you or treat you with disrespect. Who do they think they are? God? Last time I checked, God doesn't treat his people with disrespect and indecency. If you can respect yourself and other people their should be no problem for other people to do the same for you. Showing respect for other people is not that much of a hard task. Stand your ground, stand up for yourself and tell that person the way they're treating you is unacceptable.
2) Don't be afraid to speak your mind
Some people tend to be tight lipped with what they want to say out loud but instead say it in their heads, which is completely okay and sometimes it's the smart thing to do. Then there are other people who do not care what they say in their heads that choose to vocalize their thoughts and opinions. If you notice something that just isn't floating your boat, speak up. Don't be afraid to say,"Hey! I think you're acting like a complete jerk and I think it needs to end here. Right here. Right now!" Don't be tight lipped about the unfair treatment you are receiving from someone. Let them know that their actions are unacceptable and unwanted in society and in your life as well.
3) Always remember that you deserve to be treated better than the poor treatment other people give to you
Don't ever think you're being treated poorly because you're being too nice to someone or because you think you did something drastic to them even though you did nothing wrong. You have to realize people tend to act like horrible people because they aren't happy with themselves, and they think it's cute to act like a complete jerk. They have no morals or respect for other people around them, and sometimes they were treated poorly the same way from other people and thought that they have to act like a horrible person to avoid being treated poorly again. Don't be like that person who has to turn into a horrible person in order to avoid getting hurt, because we are not breaking that, "Everyone is acting like an A-hole" cycle. We are continuing it. We must remember that people who behave horribly to others are not happy with themselves internally and they have to project their unhappiness onto others.
Always remember to be good at heart to others no matter all the bad treatment you receive in your life.