He was only 18 years old and he was speeding. Murder is a bit of a stretch.
This wasn't a freeway where he was going 80 in a 75 zone. It was a normal road with intersections and many other road users going the speed limit or below. It's simply idiotic to drive 80 there, and it's very easy to accidentally kill someone when you're going that fast. (You have no time to react.)
Driver's Ed is 6 months of you being told this every single day. He knew it was dangerous. He did it anyway. He killed someone. That's murder.
(Personally, I think 40mph is way too high of a speed limit for roads like this. 20mph would be a much better. Car/pedestrian collisions are much more survivable at that speed, and cyclists can move in and out of traffic freely. But 80mph is just fucking insane.)
He is saying that murder is a stretch.
The reason it became murder is because he was found to be boasting about his speeding behavior online, i.e. it wasn't episodic or unintended. He meant to go fast, he planned to go fast, and an 18-year-old is an adult with adult responsibilities -- whether or not he is ready for them.
Knowingly engaging in behavior that risky — driving faster than any posted speed limit in the entire United States of America on residential streets is egregiously risky — and thereby causing someone's death is textbook murder.
EDIT: phrasing.