he hit the wall, then 3 things happened which killed him. The wheel came off, and smashed his head back into the bulkhead behind, causing death by fractured skull.
The wheel came off, and part of the suspension pierced the helmet just above the eye and that killed him.
I think the other one was that a shard of body work broke off and killed him.
Any one would have been fatal. Or all 3. If the wall wasn't there and it was a tyre barrier it might have been different.
Sadly, after rubens crash on the friday, and then ratz in qualy, he ironically put together the beginnings of the GPDA on race morning, to champion the safety of the drivers. He was the last to die in F1.