Your velocity will decrease as you decrease your load. That's logical.
Your accuracy can possibly be at any given velocity depending on loading consistency and the personality of that particular gun. Accuracy usually isn't as good with a full max load.
Killing power, is more about shot placement that any other factor under your control. A full max load that misses is just as good as not shooting at all.
If you can put all your lower velocity loads on target you will have a kill.
In timed competition, winners are those who can put "kill shots" on target fast. Full max loads are not as easy to follow up with for the next shot.
In real life self defense situations, I believe it's very much the same as timed competition; more rounds on target in a short time. All the "reduced" loads I've ever seen will still poke holes in bad guys.
Knowing where you loads will go with any given load development is good to know.