The Wikipedia article is extremely detailed:
https://en.wikipedia.org/wiki/OceanGateSome highlights include:
* Design inspired by the "DeepFlight Challenger" - but DeepFlight said their craft was only rated for one dive, and weakens with each cycle, and could not be used for five dives.
* Hull designed by subcontract manufacturer in 6 weeks - scarcely enough time to do much testing.
* In an early dive the CEO performed solo, lost contact with the surface ship for approximately one hour
* While diving with a journalist, lost contact with the support vessel for 5 hours.
* It was impossible to open the sub from inside (admittedly, this is only relevant if you first manage to ascend to the surface)
* Hull started showing showing signs of cyclic fatigue in January 2020 (they got it repaired)
* Assembly and testing procedures so sloppy they managed to attempt a dive with a thruster installed the wrong way around.
* Employee called for a stronger front window, and nondestructive testing of the hull. Company fired him and sued him.
* CEO on record as saying ship safety laws "needlessly prioritized passenger safety over commercial innovation" and that "At some point, safety just is pure waste. I mean, if you just want to be safe, don't get out of bed."
* Couldn't get certified by a ship classification society, claimed that was OK because most marine accidents are operator error not mechanical failure; and the standards didn't give them adequate credit for their corporate culture of safety.
The other thing that indicates it was a deathtrap is the deaths.
With that said, personally I support the right of people to expose themselves to the risk of death in search of adventure. Normal folks can buy motorbikes and quadbikes, millionaires can buy cessnas, why shouldn't billionaires have deadly entertainment options befitting their wealth?