You want to insult someone by using the term "stupid".
Which of the following do you pick, and why?
1. I think you're stupid.
2. Deep down, you think you're stupid.
3. Everyone thinks you're stupid.
It depends on the situation. I can't just pick one without knowing what the situation is.
With that said...
I'll tell someone I think they're stupid if I truly believe they're stupid or to get a reaction out of them. If it's just for a reaction, I may not even believe they're stupid, but I'll say it anyway to make them angry or upset. It amuses me.
I don't recall ever telling someone that deep down, you think you're stupid. Then again, I probably have at some stage or another. If I did say that to someone it would have been because they said something indicating that they're experiencing self-hatred or again, to simply get a reaction.
If I tell someone that everyone thinks their stupid it's because I either believe that everyone they know probably does think they're stupid, or again, I'm seeking a reaction.