[Eoghammer]

a pacifist GM is often worse than a cold-hearthed, bastard of a GM... If the player don't know that any error can lead them to death, they will be overconfident...
Players must fear for their character's life every now and then... they will think for better plan and will run for their avatar's life....
My group of players fear me as i have killed nearly each of them a few times... the last time, i got a new players the others started to list several of the multiple wrong thing they have done that have costed them death or "hand of god"
They have started as a band of loose amateurish shadowrunner that will fail half their missions and tend to become a group of fiable runners...

[WellsIDidIt]

One last note: It is occasionally a good idea to pit the players up against a fight they have no real chance of winning. Make them realize that running is an option. Combat should not be perfectly balanced 100% of the time. Sometimes you have an edge, sometimes you are at a huge disadvantage, that is the best way to keep things realistic.

This is pretty much the point where we were talking about balance and equal balance not being the same thing. The players can be outskilled, outgunned, and ill-equipped for the situation, that's fine. However, if they're getting slaughtered before they can run or even see the opposition, there is a balance issue with the GM.