So the title is a little misleading because I think it's either Hazel or Tyrian that killed Summer. Hazel did specifically say that "we've dealt with {silver eyes} before" and since it was specifically him it leads me to believe that JonTr- er Hazel killed Summer. The second person that I think killed Summer if it wasn't Hazel is Tyrian. I think that because of his excitement at the prospect of killing Ruby which I can see it as him wanting to kill the next generation of the Rose family. Well there's my idea even though by definition these are more guesses or hypotheses and not theories. Whatever, thanks for reading and have a great day/evening depending on when/where you are reading this.