Major fault, likely cause is the turbo has suffered a catastrophic failure.
The reason you couldnt stop the engine if that any diesel can run on its lubricating oil, so if the turbo fails and sprays oil into the engine it will keep going and overspeed like you seen, the only remedy is to try and stall it or spray halon or co2 into the intake.
Bad news mate, at the very least you need a turbo and a strip and clean of the intake, intercooler etc.
Though the worst news is that the overspeeding has killed the engine, not all that unusual in turbo - diesels, and not an Alfa specific issue, happens to them all.
I hope you have some warranty.
Happened to a friends Saab, 'luckily' he stalled his by driving into the wall making the car a total insurance loss.