Question: How do I cover darker/socially unacceptable topics in my writing without going "too far" or not being able to find a positive solution to my character's dilemmas?
There's no rule for what is "too far," except that you can go so far as to offend the morals of your reader and therefore lose sales.
However, here are a couple of thoughts...
1. Sometimes dark subjects are covered by having a main character who is somewhat removed from the darkness. For instance, in a story about a murderer, serial killer, sexual predator, etc., rather than make the villain or the victim the main character, you can make the investigator the main character.
2. Another approach is to have several different plots that look at an issue from different points of view or different incarnations of the issue. The darkest case might be in a subplot rather than the main plot.
3. Consider not being too heavy handed. If your social issue claims many victims, perhaps in addition to darker examples you can also include an example or two of more positive outcomes -- one victim who survives and thrives, one oppressor who redeems himself,
4. When all else fails, be authentic. Don't be gruesome just to be gruesome or to shock. That's as bad as sugarcoating. Authenticity is a powerful defense.
5. There's nothing wrong with writing a tragedy. Just make sure that if your character meets a bad end, it is either because...
a) Their own actions, failures, or mistakes resulted in their downfall.
b) You are indicting the world/society for allowing evil. (And therefore providing an example to learn from.)
In other words, the dark ending should be meaningful and not just repulsive.
Best of luck.