Okay, I've heard someone from
Tumblr that gives a good theory on Tatara's murder. While it may seem a third person may be involved in this case, it could be someone close to Tatara or knows (or working for) Evil!Shiro.
I don't know,
I did give my own theories about it, but I think the video's not showing the entire picture on Tatara's murder. It's plausible the video is doctored (judging by the high-tech setting of K) but I don't think Homura is that dumb to blame the murder on a random person.
With all that mess I just threw up, my head's starting to hurt with all these strange gaps that's left out in order to see the full picture of our beloved Tatara's murder. I apologize if what I said here doesn't make much sense.