So did you read the Mobius Policy e.g.
The group's goal is to provide players with an environment to play Elite: Dangerous in a full multiplayer world without having the experience spoiled by non-consensual PvP, ganking, domination, abuse or anti-social behaviour.
To achieve this, all members agree to abide to the rules listed below while playing online in the "Mobius" private group. Of course, the rules apply only to sessions inside the group
And continue to join the group as defined by their 'rules' in order to play with the friends in that group, or decide that as their policy isn't one laid down by the game developers it doesn't matter anyway?
Just curious as you haven't actually stated that your intention was to 'break' their rules, just that FDev haven't written such in stone

(and that there is so much 'distrust' flying around that even a harmless comment could be misinterpreted!)
I didn't join Mobius specifically looking to break the group's rules, and I haven't been there in years, but I wouldn't have a problem doing it if the circumstances called for it, as these rules are indeed just arbitrary and made up by the players and are only enforceable in that it is the group owner's right to kick you out if you break them.
Mind you I would see things differently if, before you were allowed to join a private group, the game would force you, via an in-game popup or some such, to read and acknowledge that private group's rules. It would have to keep a copy of the rules as they existed at the time you joined (to be able to handle disputes), and those rules would need to be curated by Frontier to avoid the most blatant cases that could get them in trouble for endorsing them ("no Jews allowed", "send pics first", "here's my paypal" etc )... Then and only then you could justify giving group owner more authority over what happens to rule breakers beyond their current power to kick them.
What are you going on about?
When did I ever mention "punishment" or appealling to frontier. You're making things up now. Build me a straw man and then knock it down...
You explicitely said, killing somebody in a no-pvp group is against the rules of the game. Now in my world, breaking the rules calls for punishment, but regardless it is nonsensical to think Frontier is going to endorse your group's arbitrary rules with no oversight.
As a member of Mobius - yes, you have given your agreement to no PvP. That's the condition you agree to be bound by. Feel free to look it up yourself.
I haven't. Can you prove I have? Please don't quote the Elitepve.com website, since I can all too easily claim I have never visited that website or accepted the rules laid out there before joining the group.
Ignorance is no excuse only works when you have the power to make your position the default one which others can't claim to ignore. A private group's owner can do that through the power they have to exclude anybody from their group, which I certainly don't dispute.