Well, I'm a GM more than a player, but I've done some harsh things as a player.
A favorite thing for one combat hacker was to do the Anton Chigur thing where I told a guy that if he didn't cooperate I would kill him then find his five closest DNA matches on the global SIN registry and kill them too. I proceeded to do so, just to prove a point to a dead man.
As far as how low I can go as a GM, how about this as a counter to the "Uber assassin who kills runners I don't like" concept being bandied about as pure genious:
"If only you'd killed that security guard, then at least his family would have received death benefits.
Instead, he got fired. After all, somebody has to take the blame when a valuable prototype gets stolen, and it sure as hell isn't the management.
Because of your heroic restraint, his whole family was left homeless in the barrens, where they got scooped up by the 405 Hellhounds. His wife got passed around in a five-hour-long session of gang rape, proved too unruly and was killed in a manner involving much stabbing and thrusting. Meanwhile, his kids were set in a pit to fight other kids, hardened barrens kids, for the right to get jumped into the Hounds. Their dead bodies were later sold as ghoul food. The man himself was shot in the back way before all this though. The Hounds stuck some meathooks in his body and drug his corpse behind their bikes, all the way over to the local bodyshop, to see if there was anything in him that they could salvage. Too bad for them, there wasn't. It turned out that the corp took back all their ware when they fired him and threw him out on the street with nothing but corpscript that he couldn't spend outside of the company stores where he was no longer welcome.
Next time, murder every guard. Their families will thank you."
Enforced morality is never a good look..