Been thinking about a theory that is out there and doing a bit of theorizing on the subject myself concerning "who created the xenomorphs." Since this is mostly just fan speculation, it may not need a "spoiler" tag. But there have been a few reveals from Scott that are built into the argument, so...
*********SPOILER ALERT*************
So, although Scott's exact words on the subject are a bit cryptic and hard to follow, he seemed to say that, contrary to what almost everyone thought after Prometheus, the Engineers did NOT create the xenomorphs, but we will learn who/what did. Without getting into the specifics of how, the theory among most is that David created them. This seems to be supported by the clips, interviews, and other information drops that have happened so far. Newer info has most people that I have seen comment on the subject further speculating that Shaw is dead and that David's actions in killing the Engineers, experimenting and creating the xenos, and likely killing Shaw. This also seems to be supported. ...or maybe not. Perhaps the info that has been released so far hasn't actually gone so far and revealed a bit too much after all, but was instead carefully crafted to lead us astray. So let me propose a slight variation.
Here's what I think happened: Shaw was behind or involved in it all and David didn't necessarily "rebel" at all. Why do I think that? At the end of Prometheus, she finally understands that the Engineers are out to destroy humanity, and she wants to stop them at all costs. That's her motivation. That's her goal. She manages to convince Janek of this, which is why he rams the Engineer ship and causes it to crash. She then takes off to the Engineer world (or at least, one of them). What was she going to go there to do? To stop them, of course. How would she do it? She would need a weapon. And she and David had a ship full of the ingredients to build such a weapon. We know this. And we know, from the clip screening of Covenant, that David does indeed unleash something and kill the Engineers on "Paradise." To me, it seems obvious that Shaw was thus behind and involved with the xenocide of the Engineers and the creation of the xenomorphs, even if David may have done a lot of the scientific dirty work for her. Now, I'm not sure that ultimately changes much. But it's just interesting to me that folks all around the Internet seem almost unanimous in their assumption that David rebelled and created this evil and likely experimented on and killed poor ole' Shaw in the process. I think that's too simplistic and that we are going to get a surprise variation on all that, such as her perhaps voluntarily giving herself up for experimentation, or her being alive and pulling David's strings, or something else. We'll know soon enough, but just wanted to throw this out there.