Why would somebody make a deal with an inherently evil creature at the cost of their own soul?

Because the 'evil' entity is offering what looks like help while the omni-benevolent being isn't.

>Firey McSoulfucker
I'm gonna use that name.

You live in a word where people cut their own dicks off daily for various retarded reasons.

It's really not a big stretch

Well if you, for example, witnessed your sister being reverse mouthfucked through her nape and you wish to do something with it, you won't concern yourself with repercussions. This is why damned are called damned.

The big irony is that someone willing to make a deal with the devil should logically already deserve to go to hell, meaning the devil doesn't even have to make the deal.

This isn't christianity, hell isn't a ruly place. A devil doesn't buy your soul for it to just go to hell, a devil pact binds your soul to that specific devil/demon. That's also why you make a contract with an actual devil instead of "evilness" in general, and why you can even make the pact. Hell doesn't care about you, but any soul a single devil has makes him stronger.

Apart from the absolution via redemption; yes your correct but just confess your sinsurance and ask for redemption and pop into limbo you go.

Most D&D settings, and derivatives thereof, treat souls as a sort of "Malleable Conciousness". When a mortal dies, their soul goes to an Outer Plane and becomes a minor Divine/Infernal being. Sometimes retaining conciouness, sometimes not. In the latter style, maybe the souls seller draws enough of a distinction between themselves and their resulting after-life being to just not give a shit about it. But if it's the previous, another constant of the genre is that souls can "Upgrade" themselves once they're a Divine/Fiend by pleasing their master, or just being particularly good at evil fuckery. So, they could justify their actions by believing that they'll be one of the exceptional servants who eventually becomes a mega-demon.

Most of the time folks are tricked into it. Devils are decievers and will often try to make the deal seeming like something other than the spawn of fucking Asmodeus. The deal may be vague or complicated, or made in a desperate or "heat of the moment" kind of situation. Anything to have the person giving their soul from thinking any about it and realizing what they've done.

Not very often the red horned satans will just walk up and say "I can give you a billion dollars if you"ll let me cornhole you with barbwire made of acid spiders for the rest of eternity when you die"

Have you ever heard of Faust? or Doctor Faustus if you're into plays, but the short and narrow is a dude makes a pact with a demon for hidden/forbidden knowledge; dicks around with it for years, ponders redemption through Christ to get out of pact, doesn't, gets dragged to hell by demons. Lots of philosophical debates about fate, God, and human nature to be had. Basically it's tricky and if you've made a deal with said demon just finding a way out is hard and probably not likely but you do it as a sort in the moment kind of thing. Like you want to be best at blues music, and this is the fastest way to do it.

I'm actually making a short story all about soul stealing (anima, ad iRe-label it).

Can anyone point to instances of ways daemons interacted with humans that I could recast as soul stealing even if it wasn't? (For example incubi/succubi didn't fuck people's souls out historically but it works well enough in inspired works)

If they are in a situation where what they are going to lose otherwise is more important to them than their own life.

God damnit user this is not an alignment thread !

Because there's no good-aligned beings that would grant you that power boost at the cost of your soul. Also, in plenty of settings there's ways to directly ascend into a devil/daemon/demon yourself. Sell your soul, become one of the aforementioned before you die, and boom, you're now an immortal fucker that will only grow in time as time passes.

Okay, it's been a while since I've seen La Damnation de Faust and it's been even longer since I've read Faust, but I'm pretty sure one of the major points was that his waifu gets his soul whisked to heaven.

Depends on the setting. Are the evil demons still relatively trustworthy? Will they always screw you over, or abide by the rules of the contract you sign? Does a soul have a set value?

Once, I played with the idea of a setting where you could only get magic by being given it by a more powerful being. Mortal users could pass it to each other but it got weaker each generation; for full power, you needed a god or demon to empower you. I came up with the notion of a society of wizards/warlocks who LOANED their souls for power. Signed it away, got the power, and had an arrangement with their demon to provide an equivalent service (with interest) in exchange for getting their soul back. So long as they managed that before they died they were golden on the demon front.

Surely that only happens once, not daily? Are we dealing with some kind of half-troll harem guards re-castrating themselves every day or what?

It's a legend, so it's pointless to discuss without acknowledging specific sources. Marlowe's is what's being discussed in that post, but in Goethe's take, yeah, that's totally right.