This was an interesting take on why Gellert became a Dark Lord. I loved how you characterized Gellert, especially in terms of Ariana's death.
He humanized him. You showed us why Dumbledore had been friends with him for years, what drew Dumbledore to him.
I liked the way you had Gellert thinking about Ariana, about why she had come out of her room and made sure to say her being killed was an accident.
Tragedy can change people and for Gellert, all it did was bring forth the need for power, something that I believe is realistic.
There were some mistakes (For example, you wrote 'her' when you were referring to Gellert), but overall, this was a wonderful character-driven story.

I love you, I love you, I love you, I love you! Wait, did I tell you I love you? No, well let me say it again: I FREAKING LOVE YOU!

Gellert and Ariana-! You may just get that GellertAriana fic from me. This like totally converted me into a Gellert/Ariana shipper. :) First off, I can tell this will probably be super long because I'm gushing inwardly and OH MY GOSH the emotions here were so freaking breathtaking. Gellert, a character that as you have seen, I usually picture to be the kind of cold/warm kind of person in personality, was utterly distraught over her death and I just- oh Merlin, just YES.
(I'm getting more and more into these reviews, so pardon me if I sound a little jumpy)
I've always loathed Albus- this I think everybody knows XD - but with him, I always just sort of didn't care for/like Gellert. You've completely turned me to his side with this and dang did I not say I love you for this?
The spelling and grammar were really good although I saw one little problem with the tenses.
"Farewell, aunt Bathilda," he whispered, pressed a kiss to her cheek and tried not to run as he left the cottage,..." (At the very bottom)
It should be...
"Farewell, Aunt Bathilda," he whispered, pressing a kiss to her cheek and trying not to run as he left the cottage,..."

Besides that, AWESOME JOB, CARMEN!
~Fire