"The moment I saw his face, I knew he would never warm up to me". turns out it was Rubero saying that lol. It was not the kid finally recognizing Rubero as a loyal guardian, but a tale about how Rubero groomed a kid to be his...servant?
I don't think the dog purposefully caused the deaths, more likely he is attracted to it, hence being originally found wandering in a graveyard. He is perfectly capable of killing directly if he really wanted.