The cop taking the video was speeding: 65mph in what looks like a 40mph zone. Another cop passes the cop on the right, much faster. No emergency lights, not responding to a call, just going home. A car going the other direction turns left; the second cop hits it at 94mph, killing both people in the car that turned.
Here's a thing to notice: between when we can see the turn starting and when the brake lights come on, around two seconds S L O W L Y tick by. About a second after that, not having slowed visibly, the cop car has hit the victims' car. EDIT: Google map showing the distance he traveled during this time.
The kids died because they saw how far away the cop was, but they either didn't realize he was going 2-3 times as fast as they expected or didn't understand the implications. If he had been going half as fast when they started to turn, there would be been 6 seconds instead of 3, and
- they would be been across the street already
- if they had stopped right in front of him for whatever reason, he would have had 2 seconds to notice, 4 seconds to brake, and 50% less speed to shed, and so it would have been a fender-bender at worst.
"What can we learn from this?" Being on the road and not killing yourself and other people means:
(1) Being aware of the current situation on the road.
(2) Actively anticipating what other people might do, rather than assuming they're going to do what's convenient for you.
(3) Being aware of what they're anticipating you'll do, and doing exactly what they expect. If you're going to do something they don't expect, communicate it very clearly well in advance and don't do it until you can tell they've seen you.
I'm not the cops: I'm not going to say "never go 100mph", because that's fun sometimes (or so I hear). Whatever. But regardless of whether you're breaking the rules at the time, you have an absolute duty to the other human beings on the road. Setting aside speeding, which no doubt the prosecutor will harp on because it's the part that's actually against the law, there are two pieces of terminal FAIL here:
The cop saw oncoming traffic, he saw them in the left-turn lane, and he should have hit the brakes hard and shed that extra 40mph right then, because you don't know whether someone else has correctly judged your speed or what they think they can get away with.
The victims saw oncoming traffic and they decided "eh, we can make it" instead of just waiting for it to clear. That was a stupid decision: there was nodamnbody on the road, they would surely have had their chance right after the cop cars, and you can't count on your brain's estimation of phase space parameters. But hey, they were teenagers, which means the bits of their brain that anticipate consequences are not entirely working yet, never mind that we let them fight in wars and drive cars around town.
(And this is why, if you don't leave at least a 4-second following distance when you're behind someone in traffic, I get angry: sure, you can see the traffic ahead, fine. But what about that time when someone does something a little bit boneheaded that you didn't think about? If it's their fault legally, does that make it better that someone is dead?)