Sin and it causes

In a couple of my new games that just started i have been invaded by
spirits of vengence without killing any npc's or joining any covenants.
What caused this? Also does getting killed by a Darkmoon blade absolve
your sins or can you still be invaded?

If you bbd, sin transfers, and being indicted is a sin,

Otherwise, they're likely wraith that invaded as a sov because of a glitch. It happens.

You much absolve your sins, and you cannot absolve indictment sin.