I have come across this situation recently and I believe that message you sent to unknown (not yet friend) would be in message request section. And if you delete your account (not talking about deactivated account) those messages you have sent disappeared. Even if they had already view it. Don't panic.
Yes, you have to look up Facebook's Sharing Policy in depth. If your info is private and confidential, and is being misused by the said person by being quoted out of context, this action might violate Facebook's Community Standards.
You should approach them and if you can convince them of the merits of your case , they will take down the entire stuff.
Simple download the Messenger Desktop app
from the link here :
Messenger Desktop App
You can use your existing Facebook ID to login ( and import contacts from your Friend List etc. ) and start chatting, without having all the dark stories on your Facebook feed troubling you anytime whatever.