I don't believe darkness being positive is the odd one out. Even major media has "darkness = good" not as a subverise but integral trope (see, for example, darkness).

I don't agree with your assessment of Batman and I would point out that Batman isn't regarded as evil in popular media (and even if he was, Superman was evil plenty of times, see Superman Red Son)

Lastly, I would mention that in hero types, a internal struggle is usually desired to counterbalance or embolden external conflict. Even superman has internal conflicts.