True. For now.
I'm hoping that the Fantastic Beasts trilogy will change this. For the first time in a while, we'll get a fresh perspective on the wider Potterverse through lenses that aren't Harry's spectacles. If Dark Magic plays a role, then we might have an influx of entries all over the web that are serious. Optimistic, yes, but plausible - at least for the sheer potential rise in numbers that one might expect.

 

I agree that there is already plenty of potentially catalyzing events and circumstances in Harry's life to lead him into darkness. I think any number of very subtle changes could be written into a slow domino effect over time where the Dark Arts gain a stronger grip on Harry's mind.

After any one of Harry's encounters with lethal force, fear or ambition could spur him to investigate questionable magic, and slowly succumb to the Arts. If I had the ambition and discipline to write a Dark!Harry, I think this is how it would look.

If you're going to delve into the Dark Arts, I think one of the most important things to explore is the inseparable nature of their rewards and inevitable costs. Wizards don't master the Dark Arts, the Arts master them. I see a lot of parallels to the Dark Arts and alcohol/drugs.

Many can use them without their life descending into chaos, but there are those of us who just like them too much. Almost by nature, we can't have a healthy or static relationship with them, the allure is just so strong that it illicits a continual chain of compromises or sacrifices as they are slowly and steadily prioritized over everything else in life if they're used.

In the same way, wizards who are susceptible to such an allure of Dark Arts will by nature far surpass those who aren't in their mastery, being driven by such a strong force. However, the inevitable consequences will manifest as well; it's unrealistic to have one without the other.

A lot of the Dark!Harry stories I've read either gloss over the transition, which is understandable because a high-quality one takes a lot of time and skill to write. Or, they depict Harry with increased power and dangerous spells with a completely sound mind and no noticeable consequences for taking that path. Both of which don't really do the concept justice in my opinion.