Is there any penalty for a RT that destroys another within the Furibundus system?
In my case I think the players will do their best to kill Fel in the begining story of Lure of the Expanse. So I am thinking of what I could impose if they actually manage to pull that one off.

I think it would really depend on the loyalties of the players, and whether Fel has any allies on Footfall. Since the system is a patchwork of factions and economic interests, I would at least have some plan put out for who would be annoyed at the gesture, and who would be happy that Fel is gone.

There might not be any obvious shooting afterwards, but the players might be locked out of certain docks on Footfall, or have major penalties for interacting with certain suppliers and contacts. Of course, Fel might have also been pinching some suppliers, who would be happy to aid the RT and his friends for eliminating the competition.

A third option is that a party on Footfall might message the RT and thank him for killing Fel, but really have a revenge plot in mind since the players eliminated their valued trading partner in the middle of a big deal that was happening behind the scenes...

Depends a lot on context. If it's part of a naval action then in the Koronus Expanse these things just happen. There's no law in the Expanse, and two Rogue Traders might not take hold of it as is. If you capture the Warrant of Trade and return it to his family, then you could probably just pass it off as an event that happened and tell people to move on. If you try to usurp his dynasty for yourself then there might not be retribution, but people might be less willing to deal with you at least at first.

Hello there.

There is no real law outside the Imperius Dominatus. There is only the laws of groups or institutions try to enforce.

I do not think Rogue Trader has rules for behaviour, even in Furibundus system. Of course, an attack on Footfall could gather Rogue Trader against an attacker.

When I played Lure of the Expanse I let Fel make an alliance with Feckward to make him stronger.

When I played Lure of the Expanse I let Fel make an alliance with Feckward to make him stronger.

Good idea Lars.

You could always add a rival talent. The rogue trader they killed being part of a very powerful criminal organization or the Inquisition. Or a vengeance seeking family member. As others say it varies based on the context. Then work from there.

Well, Krawkin just got captured by the PCs & Chalabelle in my Lure of the Expanse run. They gave him to her for negotiating a ransom with his dinasty/business partners.