His story seems unfinished. From the Altar quotes and from final part of Shadow and Fortune, it seems that Hecarim at least at one point had the intentions of being a good man in life. Its unknown if he became corrupted by his own desire or by something else before he committed the ultimate act of betrayal against Kalista.
“You can be free, Hecarim,” said Illaoi, her voice strained to the limits of endurance by what she had unleashed. “You can move on, live in the light as the man you always dreamed of being before his grief and folly remade you.”
Hecarim roared and swept his glaive at Illaoi’s neck.
Miss Fortune’s blade intercepted it in a clashing flare of sparks. She shook her head.
“Get out of my city,” she said.
Hecarim’s blade drew back for another strike, but before the blow could land, the light finally pierced his veil of darkness. He bellowed in pain and fell back from its burning touch. The dark rider’s outline shimmered, like two picture box images wavering in candlelight on the same backcloth.
Miss Fortune caught a fleeting glimpse of a tall rider, armored in silver and gold. A young man, handsome and proud with dark eyes and a future of glory ahead of him.
What happened to him?
But either way, I agreed. Shadow Isles needs another lore pass to clear a few more things up.