I think he was a passenger completely.
I just looked at the whole thing in slow motion:
1- He runs over the kerb and the car oversteers to the right
2- He countersteers, to the left obviously, but the back end is already too loose, he spins. At this point Webber removes his hands from the wheel altogether, standard procedure, to protect the hands.
4- As the car hits the wall, the wheel steers all the way to the right on it's own. A moment later Webber catches the wheel, but doesn't turn it. When he sees (or feels) Rosberg's crash, he removes the hands again.
From outside, at the moment of impact with the wall, his front right just lifts up in the air. His front left seems to be on the ground, but would that be enough to steer the car?
I personally don't think Webber did this on purpose. He's a nice guy and a sportsman, and I just don't imagine this train of thought going through his mind.
This is when he started sliding.
https://pic.armedcats.net/m/mp/mpicco/2010/10/28/web1.jpg
Just before impact, he takes his hands off the wheel, nothing else he could do to control it.
https://pic.armedcats.net/m/mp/mpicco/2010/10/28/web2.jpg
Already going backwards he grabs hold of the wheel for whatever reason (it was spinning wild)
https://pic.armedcats.net/m/mp/mpicco/2010/10/28/web3.jpg
Pictures of his front wheel up in the air, the left rear suspension probably totally collapsed cos of the impact.
https://pic.armedcats.net/m/mp/mpicco/2010/10/28/wheelsoff.jpg
https://pic.armedcats.net/m/mp/mpicco/2010/10/28/wheelsoff2.jpg
Also to note that the front wheels were on full brakes from the moment the car spins, but when the left tyre hits the wall it starts spinning. Probably severed the connection between the brakes and the pump and the calipers released the disk; the right front remains not turning but this could be cos at this moment it's up in the air. So from the front wheels we can't tell if Mark's braking or not at this point... but I find it hard to believe he'd let the car go just to hit someone. If he did indeed release the brakes once he was traveling backwards, it probably was with the intent of reaching the outside of the track as soon as possible and hoping nobody would hit him.
I guess only Webber knows, on this one. Perhaps if they released the telemetry?