Or letterbomb him, it sends the same message.
Couldn’t you just make friendly NPCs invincible, except if course for certain scripted sequences like when the scientist falls down the elevator shaft, or the barney gets shot by the assassin.
EDIT: Like someone in this thread already said, killing people in a video game isn’t going to turn them into violent killers in real life. It’s my opinion that if your child is in a stable loving environment, they can pretty much shoot the crap out of anything in video games without turning into psychopaths.
But CatzEyes did bring to words something I agree with, at some point it’s no longer about the rules but about establishing your (moral) authority over your child.
On the other hand, kids will be kids and you can’t always look over their shoulder, so you’ll just have to accept that they’ll shoot the occasional friendly NPC in the face or groin 
I think she’s gotten an answer that works. I suppose the only further use of this thread is to debate video games effects on kids, but since we are all gamers I suspect that debate could be fairly one sided.
Hey, who better to argue about video games than the people who actually play them! Not like those feckin’ bastards who keep claiming video games to be the devil in disguise, even though the last game they played was Pac-Man.
I’m sure Pac man ruined someones childhood as well. Like they didn’t get to socialize properly because Pac man became a substitute for real life.
Addictions and obsessions come in many forms.
Plan some time away from the computer now and then. I know I spend way too much time in front of the screen.
I still swallow white pellets whenever I see one. It’s, it’s just hard you know?
I dont need water when taking pills. Now I know why. Pac man did it to me!
Yeah, thats why so many kids listen to repetitive music and eat little pills all day, pac man
I know, they can make a code for U that plays Rick Astley’s Never gonna give you up EVERY SINGLE TIME A Scientist is killed. essentially rickrolling the poor kid whenever a scientist dies.
Why don’t you use that mod that if you shoot a scientist they turn into monsters and try to kill you. then just give them infinite health.
Edit: Also when I was 11 and my cousin introduced me to half life, I killed about one scientist then I just got bored and found it pointless and I just went on with the story/campaign, he probably will too if you don’t give him that restriction. Kids love to do what parents tell them not to.
So, replace the death sounds with duplicates of Never gonna give you up? I like it!
Of course, you could always tell him the life story of all four scientist models, dropping in similaries to his life so he’ll identify with them more:
"This is Walter Bennett, he played [sport or activity] when he was a kid, and he loves to eat [favorite food] for breakfast. He got his PHD at Queens, when he…[random information about glasses/walter head]
^ your suggestion sounds like the “Rickroll Radio” custom weapon from DOOM
Here’s what I think you should do:
Watch him as he plays HL. If he kills a scientist or another “friendly”, slap him up the back of the head (like Gibbs does to Dinozzo and McGee in NCIS).
[COLOR=‘Black’]…What? Pauley Perrette is cute![/SIZE]
Just let him play the game, it’s a definite part of Half-life and by restricting it you’re taking something away from the game.
Besides, after a few times he’ll get bored and just play the game without killing the good guys.
My father told me when I was little that It wasn’t a game but real people that I was controlling so I was killing innocent people If I killed the scientists
Catz, I wish desperately I could help you.
I like your idea, though. I’ll be adopting that rule to use with my kids in the future.
I’ll tinker around with Half-Life to see if I can manage anything. I am a programmer, but as far as I know, there’s no way to access the HL sourcecode to modify it and then re-implement it in the game.
But I’ll still try 
Thanks Dillxn. Just trying to set reasonable boundaries but I cant watch over his shoulder ALL the time.
Hmm…you could do something crazy like record with FRAPS at horribly low settings while he plays and then review the clip afterward, fast-forwarding to the parts where the guards/scientists are. Even more Big Brotherish, but it would help you watch over his shoulder most of the time (unless he figured out how to disable it, at which point you could yell at him about that).
If I were you, I’d try to code the scientists to turn into Raminator. I don’t even need to go into further detail, because that would just be creepy enough.
I think that would just give him nightmarish images for the rest of his life