Turning right on red vs a u-turn on green***

Title is slightly vague compared to how the actual scenario played out.

I'm simply curious because this just happened and I was honked at for making a right turn on red once the oncoming traffic was clear.

For perspective, it's a 4 way intersection, let's say I'm on the south end turning right, the other individual was on the east end, turning left(assumed). I say assumed because he had no signal on and ended up pulling a u turn. He pulled up behind me honking and I instinctively(bad habit) gave him the finger.

There was no way to tell the drivers intention so I made the decision that the way was clear.

Was I right in assuming so or was I wrong to have made that right turn? I'm not infallible just curious.