It's a really simple argument: suffering is the only thing that matters because it's the only feeling that exists. There are no such thing as "good". There is only ever bad, and happiness is a false idol. This is all you need to prove negative utilitarianism

Negative utilitarians agree that suffering is the thing that's most important thing and that happiness can't outweigh it. Of course, if you have a lexical threshold view then after a certain boiling point happiness begins to not outweigh it.

Given how after x amount of suffering it can't be outweighed by any pleasure wouldn't it be most logical to kill yourself? Yes I'm aware it causes a kind of suffering, but wouldn't killing yourself with a garenteed amount of suffering outweighed the potential for the most heinous torture imaginable happening to you? On top of that, given how we inevitably die would it not be better to control the matter which we do die, which can be done through methods.

The two counter I can think of to this is the potential to help someone avoid that kind of pain, but how likely is that to happen, and as well as how would the odds of that happening to you weigh against it? Perhaps if you were a police officer, fighter fighter, politican, or something of the sort Which you can be replaced fairly easily and there's no garenteed the slot you replace will be better, or if you make such a difference in such critical moments. Maybe if you became a serial killer giving people painless lethal injections in which case good luck.

The secound one is the religious one where, going on the assumption he'll automatically outweighs everything, God supposedly convicnes you he exists, however given how God is just and wants us to have a relationship with us, while being all knowing and omnipotent, if such a being exists, then our faith should be the same regardless of our actions since God would have intervened if he truly wanted us to be with him. So if it sends you to hell you can have comfort, if it's possible, in knowing it was inevitable.

Tldr: potential for unjustifiable suffering exists and if preventing it painlessly, with some emotional pain, exists it is preferable.