Just because the guards are unwilling to look into things doesn't mean that it falls on you. After all, you're not even a citizen of Riverwood. You're a citizen of ... of ... actually, you're pretty sure the small collection of huts and farms you call home doesn't have a name, but this girl's death isn't your problem all the same.
Following the sounds of your destiny's call, you move toward the base of the nearby mountain, High Hrothgar, and look up to its peak. Is that shouting really coming from all the way up there? What a hassle. Suddenly, a strange noise breaks your concentration, and what may have been a giant paw slaps you across the face.
You spill to the ground, disoriented. Looming menacingly above you is an angry-looking frost troll, ragged and fearless. Up until now you've only heard stories of the fell beasts. Perhaps this is the creature that was killing the townsfolk? Regardless, this situation is way over your head.