If that’s the case, then the chance for instant kill is likely either a flat number, or somehow decided by monster to player level difference.
Still, I’d think the attribute bonus to kill-chance would be increased by (SPR*0.X), where X is… something… 
I don’t like guessing very much.
Maximum Extinction is most likely how many possible targets that you can kill in the AoE. If you look at the video on the link that @II_JoJo_II has, you can see that it’s a large AoE. It’s probably just that. The scaling off of SPR and the % chance it has at killing something, that I do not know.