The death, ie the implosion, would have been pretty quick, fractions of a second, but due to the nature of carbon fibre I think it is likely there were many seconds or even minutes of cracking sounds as the thing started to de-laminate, and they , or at least the captain, who designed it would have known he, and they were buggered.
He was an interesting character, an aerospace engineer trying to apply aerospace technologies to submarines, a man with huge self belief and no regard for safety - interestingly its people like him who down the centuries have made great strides in technology, and occasionally, like him, paid the ultimate price for the gung ho approach.
Where he went very badly wrong was taking paying guests, with no grasp of exactly how experimental, unconventional and risky his idea was with him.
The technology he developed in titan, is actually fine for low pressure, shallow dives, and could have, as he hoped, opened up an interesting market for underwater tourism, but was never going to be safe to go once, let alone many times to the titanic.