Speed is a factor in severity, but never the sole cause of an accident. And since almost everyone speeds, speed is almost always a factor in collisions. Therefore, speed is almost always a factor in fatal collisions. For some reason Ontario sees this as causation. You could literally say speed is a factor in ALL collisions, even if the drivers weren't speeding. So if speed is a factor in 100% of fatal collisions, of course it would be the "#1 killer".
But to say so is somewhat disingenuous, and is more akin to pushing a narrative than anything else. Blaming speed is the easy answer. It gives them justification to keep issuing those one quarter of a million speeding tickets every year. Catching people for speeding and proving it is also much easier than catching people for distracted driving, aggressive driving, tail-gating, etc., and proving that. Cracking down on speed allows them to say they're solving the problem. Funny how the problem is never solved, yet we continue pointing the finger at the same culprit. It's the same kind of logic we use in our cities when a high-profile cyclist death occurs. Automatically it's assumed that we need to make our roads safer for our cyclists, rather than convince our cyclists to practice safer riding. And so we spend inordinate amounts of money, re-direct traffic, cause greater congestion...all in the interest of protecting cyclists. Yet it hasn't worked. Hmmm.