Don't be so quick to change spine. I had the same problem with my Kota, but changing spine didn't seem to make any noticable difference. I was at the point of total frustration, then I remembered something I read in Byron Ferguson's book. He said he prefers the longbow because it is harder to torque the string. My longbow shoots pretty much everything I have without a problem and it has no more center shot than my Kota.
I went out and tried rotating my hand outward slightly (clockwise for a right hander) to match it up more closely to the cant of my bow. Bingo! My shots went from 4 to 6 inches left to dead center. I had suspected it was not the spine because the arrow flight was good, but the shots went consistently left. I have to make a conscious effort because bad habits die hard, but when I do my part the Kota will put arrow after arrow in a six inch circle at 20 yards centered perfectly regardless of spine or point.
I shot 125 gr. snuffers on 2016's cut to 30", 125 grain WW on 50-55 cedars cut to 29" BOP, 125 grain field points on 29" and 30" 50-55 cedars,125 grain target points on 29" 2016's and some old Bear cedars of unknown spine with 125 gr. field tips. Before changing my grip on the string, they all grouped left. Now they all group on center.
The Kill'Um is a great bow, but the limbs are narrow. This makes them launch arrows faster than one might expect for the weight of the bow, but it also makes the limbs easier to torque. Try to line up your wrist with the string and see if your arrows don't come back to center. Mine did!