I need for my kit datapack!

So basically I'm making a datapack that has kits and stuff and there is this one I just couldn't figure out. The kit is called "reaper" and basically you have a scythe (which is a warped fungus on a stick for right click detection, just retextured and renamed) that you can use to kill anyone in a 12 block radius if they have 3 hearts or lower. I have tried all sorts of things but I can't figure it out. I hope someone knows a fix for it! All hope will be appreciated.