To summarise what Sorle said, you are both "friends" so him killing you may just be banter.
But, if you were someone random that Mystic RDM'd then I would understand this report, but paired with the fact you didn't make an F1 or attempt to get Mystic punished in a way seems strange to me.

Rather than wasting your own time you could of made an F1 and dealt with it in-game (and the fact you didn't makes me think theres something that provoked you into making this later on)