Sounds like Frodnar and Dorthe just inherited a sizable estate.
The OP apparently. My buddy tried to do that with Morrowind. Depopulated a sizeable chunk of it too.
Umm... I'm an assasin and even I don't kill Cities, towns or even villages over such minor disputes. Did.. oh I don't know... running away ever occur to you? Guards will only follow you so far. I suggest restarting. As Named NPCs NEVER come back. Even ones the vampires kill if you have Dawnguard installed. Save often, if you think an NPC you'd like kept alive has died. Revert to an old save. Same goes for getting caught, revert. Unless you are RP'ing the character then you just have to suffer the consequences.
Well, live and learn, my friend. It's just one town, without much of consequence in it. After the main quest there is little to no reason to return there.
It is always a good idea to save the extermination binges until after playing the Game a few times.
OP - Get in the habit of having more than just One Hardsave, just in case something like killing a Vendor happens.
Before I know I'm gonna commit a crime, like, kill an entire town, I always save, and then load afterwards if I want them to "come back to life".
A friend of mine has some funny stories about what happens when you go on a binge in a major city, like Solitude. However, she resets so there's no long-term consequences to the 'experiment'.
While the game generally lacks in the true RPG element of cause and consequence, you can experience it here. This is one part of the RPG experience that you can accept.
Everyone in Riverwood is dead?
Plan R: Riverwood Invasion of the Thalmor and take over the settlement, leaving nothing alive
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No named NPCs respawn in settlements/cities/etc.