Well done diesel, you're a star!!!
You're right about it being instinctive I did much the same couple of years ago and confronted two blokes beating up another one who was down on the floor. they did leave him alone but it was only after they had gone and I went up to the fella on the floor I realised he had also been stabbed!!! Luckily ambulance and police attended quickly and when it went to court they eventually pleaded guilty so saved me from the witness box.... (it was only later that day it sunk in and the adrenalin had dropped that I realised the danger I could have put myself in).
although scariest bit was going to court only for the defendant to be walking into the building same time as me...phew he didn't recognise me!!