Basically, I think they’re assholes. They have a monopoly, they like throwing their weight around, and they can see the future where Paul Atreides is a threat to them. Sorry, I mean Moonicorn. But they’re cowards so they bully the emperor into doing something about it for them. They don’t really need a motive per se, aside from being on a power trip. Political commentary through the absence of any comments, I guess?

That said, you’ve got what? The transport guild is actually run by sentient, biotech stargates, yeah? So in order for you to go to another planet, one of the bosses of the guild has to eat you and shit you out. Which also means that every time Moonicorn goes somewhere, he’s rolling Revolution Now! Every time he goes to another planet, somebody’s like “Moonicorn! The guild ate my son!” or “We don’t have enough money to escape our war-ravaged planet because the guild’s prices are too high!” or Moonicorn notices the guild has no defences against the space madness and he can totally tell other people about that too, and then they’ll know and not trust the guild. So they really do have good reasons to assassinate Paul! I mean Moonicorn!

Starting these guys off with a bang might be kind of hard though, especially since they work through other people. If you wanted to use the Hammett plot-mover, it’s some other faction that attacks them, and then Moonicorn has to question them to find out the Guild hired them, right? “They said you’re the kwisatz haderach or something. Look, it was a job, nothing personal! Just let me go!” But it’s a game so of course it might go completely some other way and Moonicorn’s off on a wild goose chase.

Another option you have, if you’re cool with being super improvisational, is make sure Moonicorn is using one of these gates real early on. Maybe even start the game with Moonicorn fleeing another faction through the living stargate. Then roll for Revolution Now! and base everything about the guild and its relationship with Moonicorn on that one roll. Use the immediate situation and the input of the players to interpret the results of that roll, and then stat up the consequences in between sessions.