Scythes reap grain.

The grim reaper uses it to reap souls.

Linking the agricultural cycle of planting, growing, harvest with the human lifecycle is a very common metaphor.

We see this in the myth of persephone, where going into the underworld and returning (ie death and rebirth) is a metaphor for the harvest cycle. Greek mystery cults were all over this too and it's likely a pre-greek remnant from the Early European Farmers who inhabited the area before steppe-descended greek speakers began moving in.

So the association of death and harvesting probably starts fairly early.

We are the wheat.

When scythes were invented, they were a very hot, new, efficient tool. Previously they only had some curved had knifes and had to harvest every few bits of wheat individually, but with a scythe you could take those wide swings and get lots of wheat at once.

Think of it as going from a handgun revolver to a machine gun, except in a way that a medieval peasant would understand.

As ye have sown so shall ye reap!

grim reaper, Death also has the touch of death, they can instantly simply by touching or imagining it. also other things, could be controlling the forces of nature