I have a theory that I think fits. I'm not saying that I would put money on this being what happened, but it does fit.
I think someone wanted Missy dead. I don't have any theories on who, but I think just about every possibility has been offered up on these threads already. Take your pick!
That someone didn't want to do the killing personally, though. So they paid someone to do it for them. That does not mean that it's a hit by someone who knows what they're doing (someone mentioned a hit being unlikely because Missy died from head wounds and that isn't like a hitman, but I don't think a hitman needs to be experienced or skilled, they just need to have been paid to kill someone).
That someone took their money and ran. They could be anywhere now, anywhere in the world. We still don't know what that person in the SWAT gear looked like (so experienced or not, they were pretty smart), so they can still move freely around the world.
And I think whoever ordered the kill on Missy is probably hoping that because they likely transacted in cash and in person (away from cameras if smart, but we'll see) that it will never be traced back to them.