Depends on what rewards they get immediately. People do a lot of things in the real world that have immediate gain, but long term negative effects. There's also the issue with people often thinking they're special and can sidestep these negative effects.
Sure, that works for "Deal with the Devil" scenarios, like Faust, but it absolutely does
not work for actual "worshipping an evil god" or sustained devil-
worship or the like.
For that to work, you need the followers to believe that they are actually going to get an eternal or nigh-eternal reward. This is especially so in 90% of the D&D worlds, where the existence of an afterlife and the soul is pretty much completely demonstrable and accepted, and not even silently questioned.
The simple answer is that worshipping an evil being should in no way send one "to hell" or any equivalent thereof, unless it is to rule over the peons who get dragged there or something. There's no particular reason that worshipping Baddy the Badgod should damn you, unless the Christian God is in charge of the entire pantheon (which is kind of the latter-day FR situation, sadly, bloody AO - he even has a lame real-world-invoking name - AO i.e. the Alpha and the Omega), rather it should just send you to Baddy the Badgod's version of heaven, which maybe some really scary stuff, probably involving lording it over stolen souls or servitors or something, or constant in-fighting in a way which seems enjoyable (I mean, the Vikings had Valhalla, which seems like a permanent bad Friday night to me, but apparently they liked the concept!), or whatever.
The problem is, it absolute is a competition, just like with real-world religions. Christianity succeeded so extremely massively because it appealed to the downtrodden masses - the slaves and serfs - you didn't need to be important or powerful or skilled or of a specific ethnic background to get to heaven, you just had to follow the rules, and anyone could join, and the rules were about being nice to people, largely, not doing crazy stuff. And your get to leave in Heaven forever! Nice! Plus you can't even be locked out by previous sins! They can be forgiven! Awesome.
So when Baddy the Badgod only offers you a chance at heaven, and only if you like, conquer a kingdom in his name or some shiz, he's not going to get a lot of takers. So he needs to up his game. Anyway, I could go on like this, but I'll TLDR it:
TLDR: Deal with the devil and long-term-worship of a scary god/demon are not the same scenarios.