From what I have read about that sub this is almost a Darwin Award scenario. That thing was a disaster waiting to happen.
It was completely uncertified, and the divers knew it and signed a waiver acknowledging it. An ex-company official said the port was only rated to 1,200 meters. The vessel was not depth-rated by any competent authority.
The sub was a 5" thick carbon fiber tube with the single acrylic port. Inside it was just an empty can with 3 video monitors, and controlled with a video game controller. It had a single button, an on-off button. That was all. No "E-stop" even, or a mechanical fail-safe that would drop weights and return it to the surface in an emergency.
Real DSV's go through rigorous testing and certifications, and have appropriate safety margins and fail-safes. The head of the company (who perished on the dive) had previously said regulations in the industry were strangling the underwater tourism business.
There are all kinds of regulations that apply when boats hire out to take people on the water. These guys just ignored them all and wrote their own rules.
And it seems the people who climbed inside were aware of all this, and did it anyway.