The edgelord DMs who say stuff like this tend to forget that D&D is in many ways meant to be a better world. Where slavery exists, for example, it exists as something that evil people do and heroes stop. If you’re participating, you’re not a hero; you’re the asshole the heroes are there to stop.

Bigots exist in our campaign, but its because we utterly enjoy putting them in their place.

D&D generally is a game of heroism and hope. D&D’s hells aren’t the hell of out world, nor do devils serve the same role. Different settings have different themes (the style guides are useful for insight) but overall, heroism matters.

And if one likes and gets power enough, one can even descend into the hells to punch the devil himself in the face.

Depends! 5E is broken at higher levels so rarely there. I’ve had a few complete campaigns in older editions though; a group with insanely high levels completed the Throne of Bloodstone and another custom campaign closed out after saving reality itself. As for killing gods, once. One of our PCs ascended to godhood too. For the hells, that’s never been an overall goal. Freeing good souls, yes.

Maybe in your campaigns, but that’s absolutely not how the FR are designed. But, don’t take my word for it. Per the official FR style sheet:

“The Forgotten Realms is a hopeful setting. The good guys will eventually win. … While not every moment of a story or image in art should be hopeful (the villains need their time in the spotlight, and bad things do happen), keep this tone in mind.”

In the Forgotten Realms, there are nine layers of hell. What the domain of an evil god is like are as varied as the many gods. But they’re not designed for punishment; what sense would that make?

They only just recently made an adventure catering to high level characters. 2E, 3E offered stats, so clearly they intended there to be a path.