The school cop gave me the same advice when I was 17.
I was involved in an incident at High School where I was attacked with a knife by an older student. (Fortunately, this was before the daze of "zero tolerance," so I did not get into trouble for defending myself against an unprovoked attack with a deadly weapon.) It was a situation where he came up and picked a fight, and as he was inciting me I knew he had a knife. (Why else would someone push you with his left hand while keeping his right hand over his back pocket.) I tried stalling things (I wasn't crazy enough to take someone with a knife on bare-handed if I didn't have to), trying to get a teacher to notice and break up this big fight brewing in the cafeteria, but suddenly a lot of them found reasons to be somewhere else (maybe they figured he had a knife, too, or maybe they were afraid of getting sued). So I had fifty witnesses that stated that I was trying to avoid trouble.
Didn't matter. This clown hauled out the knife and started swinging. He didn't know how to use it so I blocked most of the blows, but he did bounce one off my ribs.
That was when I uncorked on him. I knew no one else was going to end the fight, except one or the other of us. I grabbed him, threw him against a wall, then jumped on top of him. The fight ended with my hands around his throat, one knee on his chest, the other on his knife arm (he had dropped the knife, but I didn't know it), and I was banging his head on a concrete floor. It took four students to drag me off of him, or I would have killed him. As it was, I dazed him so badly that when the school cop came by five minutes later, he was still on the floor trying to figure up which way was up and not succeeding. (Because of the way he was swinging at me, everyone assumed it was a simple fistfight. No one saw the knife, and no one saw any blood until I stood up. All my injuries were on the front, not visible while I had him down. Everyone freaked when they saw my shirtfront soaked in blood, and I got dragged down to the school nurse with the rest of the lunchroom crowd following.)
The kid that attacked me was a real bad boy. He and an identical twin brother had kicked up a four-year crime wave in my home town. One would rob a gas station, liquor store, or stop-and-rob, while the other would be at a party. Then they would play the game of good-twin/bad-twin, and there was always reasonable doubt when it came to trial. You cannot imagine the cops' delight that they had one of them, caught, literally covered with his victim's blood. He was 19 when he attacked me and they sent him away for a looong time.
During the trial, the school cop and I were alone together for a while. I asked what would have happened had I killed the kid. He told me that I would be the one in the dock, but that I would almost certainly get off because of self-defense. The only downside would have been I would be the one paying for lawyers.
Then he went over the rules concerning self-defence in Michigan (where I grew up), and finished it by stating what that sheriff said almost verbatim.
I wonder if they teach them that in cop school, or if it just instinctual?
When I was a teen a cop gave me this advice "If you want to kill someone, don't shoot them. You'll go to jail. Just run em over with a car and say it was an accident. the most that will happen is you'll maybe get a year for negligent homicide".
Of course, having said this publicly, it'll be hard to use that excuse again.