In my eyes yes, you would be a terrible person if I knew about it – but you are not a terrible person to me just for asking this question. The fact you call it “terrible” shows that you recognize the nature of your behavior and find it wrong. You are not acting according to your conviction.
Picking who will I be nice to and who will I be terrible to is terrible in a way. Drawing the line, dividing people into categories and then treating them differently – that reminds me of some of the worst events in human history. The problem is when you know you are terrible to someone and yet don’t change your attitude.
It would be a different story if you asked “I was terrible to few people…” Recognizing that you are terrible to someone and already calling it terrible shows that you think about how you behave and that you care. It may not be enough to change but it is definitely the step in the direction which allows for a change.
Sometimes I wonder why people say “I am so terrible” or “I am so lazy” and laugh at it as if saying it was making them less terrible, as if it was some kind of excuse. If you know you are terrible and still don’t want to change, what is the point in highlighting it? Just try not to be terrible.
Look into yourself, try to find out why are you terrible and change it. And make sure you really think it is “terrible”, maybe you’re just worried that your behavior could be considered terrible but in your eyes is not. That would be for another question.