Let me break it down like this: I don't want to play bad guys. I don't want to play "tortured souls." If I play a less-than-good character, it's because of irresponsibility, not angst. So you tie, and continue to tie, the warlock to that flavor, and I continue to not play the class.
No big deal, right? Hey, look at the paladin. Except . . . well, there were some rumblings about non-lawful good paladins being in the base game. And the paladin's powers are pretty neutral, cosmetically. Only minor tweaks necessary. If everything about the warlock is dark and weird and demonic, fixing it will take a lot more work.
Besides, if I am going to include angst in a character, it's going to be over something the character actually did. Worrying about whether intrinsic traits are inevitably damning is so 19th century.