"Now, you've got to tell your farmhand to start farming, by clicking the command button. If you turn farming mode on, you can restrict it to a certain range from where it is standing at the moment - so lead or push it to the center of your intended farm first." (Farmhand wiki page)
Using this type of npc i found them annoying when you have to put them in farm mode. You bring them to a point and try to change mode but they move away from you, so the center of the range (the point where the farmhand is, for now) move to another point. So i suggest two alternative corrections:
-The center of the farming range is where the player stay when he interacts with the farmhand, so you can easily decide where it is. This would be the best case.
The alternative is:
-The farmhand follow you to the exact point where you are (or 1 block of distance), and when he's there he doesn't move away from that point (or walks in a very very small range around the player, max 1-2 blocks)
didn't want to add this topic of slaughtering in the suggestion. i think that this npc would be too much stupid for that work and kills too much animals. However i can suggest you to use morgul flowers in animal farms, they bring the animal health to 1 so is easy to kill them (this is something an animalist should not read).