Say you're moving in a team of randoms...

[kadrin]

Depends on the distance and threat involved (discounting involuntary shots to the back because of timing).

If you move into my sights when you're only 2 meters from me, and I'm aiming at some guy 100+ meters off, I'll just get into cover and look for other targets.

Do the same thing and the guy I'm aiming at is only a few meters further up, and is an immediate threat to my own survival, you're getting full clip into your back as I kill both you and the other guy. Your own fault, at such close distance you should be aware of where your teammates are and where they're looking.

If you move into my sights and you're 100+ meters away and so is my target, there's a damn good chance you didn't know where I was aiming, so again I'll just go into cover and look elsewhere.

[indirect]

Depends.
Greenies (zerg ones) are generally pretty bad and I try not to run with them. If they get in my way for too long I'll take the hit and kill him.