So roleplaying an impulsive or even psychotic character is outright wrong and unacceptable? Especially Shadowrunners are known for their "professionalism" and "high ethics". Our GM had many people we knew we wanted to kill in our games. And we ended up killing most. Some because the plot dictated so, others because he was an intolerable asshole/someone doing wrong stuff so we didn't like him. So it screws up the main plot? Well, maybe you should have a safeguard in place to keep the plot from completely derailing or moving into something different. Unless we, as gamemasters, only want the players to be along for the story that we tell.
Choo Choo.
Well, no. Roleplaying an impulsive and/or psychotic character - and when you have someone 'impulsively killing someone', they are by definition both impulsive
and psychotic - is fine, if you want to play a couple games worth of combat, as the organization that the guy belongs (belonged) to - whether that's the city guard, Lone Star/Knight Errant, Saeder-Krupp, Aztechnology, the Yakuza, or the rest of the kid's gang, whatever - hunts the character down and makes an exAMple out of him. If the GM doesn't 'set safeguards in place' - which really should be understood from the get-go, i.e. the above 'peoples is gonna weep in terror when dey remember what happens ta youse ', thus highly recommending that the characters don't play impulsive psychotic characters - then the game is going to quickly devolve into the other players scrambling time after time to keep the impulsive psychopath alive. (And then the other players will decide whether or not
that is worth it.)
ICA = ICC: In-Character Actions Result In In-Character Consequences. In SR, the PCs
never are the big guns; they get through life by being either too hard to find or not
worth finding; punishing someone who randomly, and for no apparent reason besides 'I thought he was an asshole' (which really
is random), kills your guys is
not something not worth following up on. That's simply the nature of the SR world, just like it is the real world.
Now, 'this is a bad guy doing bad things to good people, and he's hired you to do them for you, and he's an asshole about it' - turning a team of runners against you - that's not 'don't care about the plot', that's 'we're a team, and this guy is a dick AND he's doing bad things, and we have SOME of our souls left thank you very much, so put a few ounces of lead through his brainpan and let's go home'. That's changing the
direction of the plot.
In regards to your 'there was this guy in the room, and I got paranoid so I killed him, but he was a bad guy so it was okay' example, that's purely a matter of luck. Or, you know, GM plans, or letting you get away with it; you killed the mole on impulse, so The Plan went ahead without some of the initial complications. Sudden random killing of an unknown person in Shadowrun - when it comes to the PCs, that is - is not generally something that should be rewarded, though. At least not IMO.