CallMeChloe

So basically, I was playing with my friend on her server, and this guy by the name of Rsdbnt, joined. It was then, we realised we forgot to private the server. Anyway, Rsdbnt was acting fine and all, and even found himself a diamond sword. I did question on why he joined, and he said he joins random servers and help players in survival worlds. I genuinely thought it was a kind person, so I let him be. Little did I know, he killed my friend, took her stuff, killed me after her, and then left the server with our items. We didnt have much with us because we just started the world so... I am pretty pissed at what he did, because we kind of have to start over. Is this considered breaking the rules?