At that depth, the pressure is quite intense - around 6000 psi. That means 6000 pounds pr square inch - that's almost a Ford F150 truck pr square inch, everywhere. And now imagine how many square inches the surface area of a human is - especially your upper torso where your lungs are located.
The sub (hull) is made of a carbon fiber and titanium mix - and I'm not sure how that would react, if it buckles / collapses like regular metal, or if it simply shatters into millions of pieces like glass.
If the sub just collapsed / imploded into itself, well - that's that. The crew got crushed to death in an instant.
If the sub explode, then that would be a very violent reaction. Probably enough to kill them, purely from that - but let's say they don't die instantly from the crushing influx / wave of water:
Air / gasses in the body would compress significantly, if not allowed to exit the body. Your lungs would collapse in an instant, and your chest cavity would collapse on itself, until all air has escaped, and then replaced by water. Your ear eardrums would also rapture in an instant. With a severely collapsed upper torse, which would happen in an instant, I think your heart and major arteries would also become destroyed in an instant.
All that space would instantly get filled up with water.
I personally think that the violent process would kill them instantly - as in milliseconds...and then when all air has escaped the body, water would fill that space, until the pressure has reached an equilibrium.
EDIT: I personally don't think they suffered. The sub likely imploded in an instant, without little prior warning (noises) if the material behaves in the way I suspect it does. Just lights out, and that's that. Brain didn't even get time to react.