That is a possibility, but I was trying to keep my tin-foil hat on the desk. If I started going down that road, we'd have a list of suspects 10 LYs long. So I kept it to people and organizations we could actually verify exist in the here and now.
But.....on the subject of other aliens, I keep going back to the Fed President Halsey talking about being interrogated by "Super Intelligent Beings" when Starship One was lost. How she had a "conversion" to some new ideal or belief and saw Paradise That doesn't sound like Thargs to me. If it was we'd be seeing all kinds of other people returning. Who or What she saw and talked to is a huge question in my mind. I also got the impression from the articles at the time that these beings were more peaceful in nature, so killing Harold doesn't seem to be their style.
But then what happened to Starship One ? They found wreckage, and that's not 'non-violent'. Unless Hudson had a hand in the whole thing. Hudson I can see being excessively violent. Letting Halsey live, brainwashing her into seeing aliens, and turning her loose to politically self destruct doesn't seem his style at all.
Guardians ? It could be Guardians. If their structures are still here, and working, I'd say it's a good chance there may be a few alive or maybe in some type of cryo-pod device. But why kill Harold ? If they're removing a single person, it would mean they have a very solid understanding of our political/social structures and it's implications. Which means the Guardians are here, and have been for a while. Once we start in on that train of thought we're bringing in "The Club", the Illuminati, the 'Real' Dark Wheel, and a bunch of other fringe theories. That's a very deep and dark rabbit hole...
No.. I am betting it's something/someone on the inside of the Empire. They would have more to gain than an outsider. Using Tharg tech doesn't mean a thing. Heck, we can buy Guardian tech on the open market. Tharg tech should be available to someone with connections, and with limited suppliers, would tend to send a message to someone who knew what it meant. Maybe the next message is ten ships with the ability to emit a shut-down field and do a witchspace interdiction. Imagine getting pulled from witchspace, finding yourself facing 10 hostile 'Battle Cutters' and getting hit with a shutdown field ?
, relieving that much more of the burden of holding your nose up high, you know, like real Imperial Royalty must do.
s! Come now! What's all the withering, gents?! 
