aJohnson said:
Depends on how "in the wrong" they were, and how hard of a time they are getting.
Also maybe, whether you actually witnessed the incident that sparked it off - otherwise you don't know who's in the 'right'. (The issue being that even if someone transgresses, it doesn't mean someone else can attack them over it).
If I'd seen someone jump a red light or cut up on the pavement and get aggro, I probably join in telling them they were a prat. Otherwise, I'm not sure I'd want to intercede and get myself thumped (physically or just verbally) in the process.
I suppose if I saw someone having a go at a cyclist who looked really upset and scared - a lady perhaps, or a slight looking chap I might assume they'd merely made a mistake and that the driver was being unreasonably harsh about it, but I'm honestly not sure what I'd do about it - it would depend so much on circumstances. I like to think I'd step in and do something, but I'm not sure... If the cyclist looked like they were holding their own and giving as good as they got, I'd probably keep out.