It was posted recently that the campaign works like reality but with magic added, and rules to help us adjudicate the action.
According to your quote, in your campaign gravity does not work like in reality. In the real world, masses attract other masses, with the amount of mass and the distance between the masses determining how strong the attractive force between them.
Where in the equations is the part where what thinking creatures want gravity to do actually changes the way gravity works, such that gravity does the opposite of what you want?
Falling does 1d6 damage per 10 feet fallen, to a maximum of 20d6.....unless you want to fall? Because if you want to fall then you die without any damage at all (because you didn't roll any), the universe just decided you should die.
Knowing that, the PC pulls the lever and the floor drops away and the mooks fall into the chasm. He wants them to fall, and experiment has shown that wanting to fall results in auto-death, right? No, gravity knows your motives (somehow) and hates you! It will always do the opposite of what you want it to do.
RPGs aren't just one thing. They aren't just a game with rules, like chess. But they are also a game with rules, and the referee has the responsibility to be a fair and impartial judge.
How can your players trust you? You have the laws of nature themselves change how they work in order to screw them over.
How exactly am I screwing them over?
If you pull the lever, and the mooks fall, we resolve it by the rules.
If the mooks pull the lever, and your character falls, we resolve it by the rules.
If you are playing a character with 200 HP and you dive head first 200 feet onto hard rocks because you think it is funny and 20d6 is negligible damage to you, then I will give you fair warning that doing so will kill you, and that you may choose to do something else if desired. Same if you have your character stick their head into a guillotine and decide to drop the blade, expecting your bare neck to stop the blade. I will warn you that this action will result in your character's death and give you an opportunity to choose a different action. If you persist, then your character will die.
As I see it, HP aren't a force field or ablative dermal reinforcement. They're the responses you take to protect yourself as well as various other factors such as luck or divine protection. There's some toughness there as well, but it isn't supernatural.
If you fall, then you do whatever you can to save yourself. Luck and higher powers may also intercede on your behalf.
If you dive for the lulz, you aren't doing whatever you can to save yourself (quite the opposite in fact). Luck and higher powers also won't intercede on your behalf because you're displaying grandiose hubris. It's one thing for a god to intervene in some small way when their favored champion is in danger. Quite another for that champion to dive onto rocks demanding divine assistance.
Hence, your HP won't save you. Also, perfectly consistent. As I stated earlier in the thread, there's a difference between combat and suicide. You can't expect the rules for the former to apply to the latter, and there is no inconsistency in that distinction.
I fail to see how the player is getting screwed. I communicate what the outcome will be if they continue with the fatal action. They have an opportunity to do something else. If the DM tells you that willingly cutting your head off with the guillotine will result in instant death, and gives you a chance to not drop the blade, and you choose to drop the blade, you are in fact screwing yourself. The DM didn't do anything beyond verify that you were making an informed (albeit suicidal) choice.