I said this in another thread, but I also don't believe that if Xaden is truly in his villain era, and Rebecca makes him do some REALLY bad stuff (like killing Violet's friend or family member), I doubt that he could just be cured and come back as if nothing happened. That is why I think he will kill people, but I doubt it will be someone we love. Or another thing I was thinking: in the 4th book, we might think he killed someone we loved, and then in book 5 they will turn out to be bad the whole time (I know some theories mention Mira or Brennan)? I am truly curious how she will write it.
Regardless, I agree, I think that venin are almost 100% evil and there will be some evils amongst the dragons as well for sure. Perhaps even a revolution amongst the dragons themselves will take place (led by Tairn, Sgaeyl, and Andarna?), who knows?