Well, look at it this way. There is a perfect world and there is the real world.
In a perfect world, they would never call you for an interview if they'd already made the decision and they would have read your mind of good intentions and approved your visa.
Sadly, we don't live in a perfect world. In the real world, the embassy is only as good as the staff in the embassy. Some of them make mistakes, some of them are absent minded. Maybe some of them don't really care.
It is possible that you weren't actually denied until after the interview but somebody used the wrong date stamp. It is possible that a clerical error caused you to be asked for an interiview although you had already been denied. It is possible that they already denied you and called you for an interview just because they wanted to waste your time as well as their own. You seem to want to believe that. Personally, I think that is rather unlikely.
In a perfect world, you would sue them and they would apologize and give you a sum of money in compensation and approve your visa but in the real world, they probably don't even know what happened or why. You can complain to the embassy, you can even write to the minister and complain to him but that will not necessarily help in finding out what happened. If you are right in thinking that the staff are horribly mean people and did this to you on purpose, how do you know they will not blacklist your name and lose your PR file when you apply?
Do what you want. Personally, I think you should let it go and move on.