While Gellert was a dark wizard, he seemed to have a somewhat moral goal of ruling over the muggles 'for their own good'. Dark wizard in this case may just be someone who went against the status quo, since even Albus doesn't know who killed Arianna (and the fact that she died means they were all likely throwing curses around). Chances are he never went deep enough into the dark arts to learn how to create horcruxes... Plus, I doubt any sane person would be willing to do it in the first place.

This, on the other hand, is a damned awesome idea. Only change the time a bit so that Gellert tries to retrieve the Hallows after Albus' death, not Voldemort's. It's not post-Hogwarts, but it's an interesting premise.

 

I had always felt it to be an important part of Grindelwald's character that by the end he felt remorse. I thought it explained Dumbledore a lot, for a start.

 

That's all well and good, but you'd need to come up with a way to explain Harry seeing Tom, AK Gellert in his Nuremburg cell, after tracing the wand's whereabouts and how he Got out of his cell in the first place.. etc.

Fake vision ala Sirius/Ministry?

True.. but would he feel remorse for every single kill?
I only put the Ariana thing up there, since it was such an important death.
It doesn't necessarily have to be that particular one though. He's as old as Albus after all, and practically started a war afaik - got to be plenty of killing there.
Maybe in his remorse, he offed Hitler himself, and knowing the jig was up, used Hitler as a Horcrux murder?
Being buddies/rivals with Albus.. I don't think it's that much of stretch that he would want to get the last laugh.. or maybe a penance thing, where he lives forever in his dank cell, all self-flagellation n shit.