Ok so a few things, I believe that everyone involved (The murder, and those that died either directly or indirectly due to his actions) was reincarnated. Even if they don't have memories of their past life then that doesn't mean they aren't reincarnated (This could be that they don't all have the same privileges or they suppress their memories). The sage I think was the psychopath, we already know who MC is, I have a feeling that the first girl that got killed is the one that follows MC, and either his sister is the hero or they want us to think that the hero is the sister. As mentioned it is possible to increase the number of jobs and he has because he has obtained dragon king as a job, but I'm not entirely sure about titles. I think that the reason he was reincarnated as a villager was so that he would go undetected in a sense. Because we know that the sage is a psycho and that there has to be some organization working behind the scenes. Though to be honest this is just a lot of conjecture.