There are a large number of reasons why a character, even a neutral one can become and stay an adventurer. I choose neutral, because barring alignment restrictions forcing otherwise, most people are honestly neutral. They lack the conviction to do true good, they are not sociopathic to do real evil, and lets not get started on Law (really Order) vs Chaos.Yes, I can agree with you on that. My main issue is that I've grown away (or evolved, whether that's progress or not) from much interest in playing a Neutral character. That lack of conviction to really do anything Good (or still shy away from Evil but might do it if pressed) unless motivated by some outside force simply does not appeal to me any longer. I've effectively chosen a side on the Good/Evil spectrum and have turned my attention to Law/Chaos as a more interesting, if even more complex, dynamic to unpack.
I suppose I've developed a taste for heroes. I've seen enough villains and fence sitters.
I wouldn't normally play a villain, usually because that would be at odds with the party. But I wouldn't call my neutral character fence sitters. It's more that the specifically have no compunctions against doing the occasionally evil, but incredibly beneficial and efficient acts that reach a means to an end much better than an actual good character could.
So while the character is normally going to be nice, and might even appear to be good at first glance, it's that moment in the backroom when he decides that yes, he is going to torture the evil henchmen (perhaps to death) to get the information needed.*
*Let's disregard that torture doesn't actually work, this is just the simplest example I could quickly think of.