Can we use the stem (not the third-person singular) to form wh-questions about the subject? if the answer given is hunters kill animals and were asked to form a question do we say who kill animals? or who kills animals?
You can use either the singular or the plural depending on who you have in mind.
If, for example, you thought that the answer might be the clans of Somalia you would probably ask: **Who (are the people who) kill animals.
But if you had an individual (or even a class of individuals in mind) you would probably ask Who kills animals.
In the example you give, the answer depends of whether you consider hunters as numerous individuals or a single group.