Yeah, you are right, beside what is official, we could guess the rest. The player could be some sort of weapon that works on "instinct", whose purpose is to purify the world. The player could be an adventurer, someone that was just teleported/summoned with or without a mission, a divine being that could grow stronger with each fight and bit of knowledge they aquired, or it could just be "you" in hopes of doing the "right thing" (based in the fact you can also further screw things up if you decide), etc.Not really, my friend, Terraria doesnt need any lore more than your "headlore"
For me it is simple, the world needed a champion, and that is you, to stop the imbalance of light/darkness. In the process you screw it up releasing "all" the spirits ala Pandora's Box killing the guide, who doesnt know he is a chaos embodiment of sorts. Then you realice how to make a good end of it. Or maybe it was the only way to restore it, to unleash it all and make a new world order. If the guide and the guns seller knows it and how much, I dont know (based in their lines about the vodoo doll)
Maybe, of all the NPCs, you are the "Builder NPC" and after all the suffer about how the corruption is eating your wonderful world, that make you angry and learn how to be a warrior/ranger/magician/summoner/mix and decide to change it yourself.
For make it short, Terraria doesn't need any lore at all. Is up to you what you want to think.
PD: The guide said to me yesterday if I saw some lights in the sky, and that was to celebrate my current victories and the next ones. That made me think if they release them after I killed the Eye of Cthulhu or not. I am not sure.
Well, we can only make our own theories.