I am not sure if this is the case, but can we use the idiom "set someone up" before there's evidence that X betrayed us, or is it something we can only say after the signs indicate that X betrayed us. In the dictionary, there doesn't seem to be any hint of that implication, but I've never seen it used in a different situation. Is it a synonym of "betray" and can we use it in any situation where we can use the word "betray"?
For example:
A: I don't think he set us up. Why should we believe that?
B: Look outside, there are SWAT people coming in through the fence.
Still believe he didn't betray us?A: Oh, maybe he did set us up.
Best Answer
Yes, it's ok to use without knowing if the betrayal has really happened.
I think it's even more common to use that phrase when speculating that a set up might have happened. So, the typical case could be that the speaker doesn't know for certain.