Another reason I'm evil adjacent is because I like monsters. Or at least, some monster races. Their existence doesn't necessitate wiping them out (well, depending on what they are). And I'm willing ot ally with monsters or evil x if that means reaching a goal.
And some of the cooler gods are inevitably not nice.
To echo some others above, I'll play evil if it fits the concept or is required for the class I want to run e.g. Assassin, Necromancer.
And, given the right situation, I've no qualms about having said character(s) screw over others in the party, particularly the goodly types* if they've already shown themselves to be after my hide.
* - there's one or two players in our crew who take a very black-and-white view toward alignment, so my evils often have to run a "detect evil-it's evil!-run it through!" gauntlet. Last time this happened, however, the "good" character got run out of the party in response and they then revived my evil one; so there might be hope yet.
My aforementioned thug, Mr. Bones, had an interesting relationship with the party Wizard.
In a "Ghosts in the Darkness" scenario, Mr. Bones suggested the Wiz be drenched in blood and used as bait. The party AND the town agreed. He didn't take much damage, and we got the maneating critters. Wiz & Johnny were the heroes! And the town marked that day to be celebrated in the future as the "Festival of Blood."
Later on in the campaign, when we were surrounded by a wall of flame, the Wiz and Mr. Bones were investigating a portion that seemed a bit less intense. To test the safety of passing there, Johnny pushed him through. He survived almost unsinged- Johnny called the rest of the party over and they all proceeded to exit...
[Dark Helmet] "Because Good is dumb." [/Dark Helmet]
Also I look back at my collection of 80's era action figures from when I was a kid, and I pretty much have almost all of the bad guys and very few good guys.