For culling methods, I prefer to cut off the head. The body will flap around and look alive, so being able to see the head sitting there separate is reassuring to me (yes, I really did get it off. The bird must be dead.)The question is how is the most humane way to do it,
I use a machete or hatchet, and a chopping block.
Some people use limb loppers (giant scissors things, sold for cutting limbs off of trees.)
That one is hard to answer, because either way could be a good choice.and do i need to pen the rest of them up to keep them safe, or continue to let them free range and hope for the best.
If you build a run, there is the cost (money and time), but then you know your chickens are secure.
If you continue to let them range, they might be fine, but they might not be.
If the chickens are ranging freely, predators are more likely to take one this day and one another day, instead of killing the whole flock on the same day. That could serve as a warning that you need to lock them up for safety, but without having to replace the whole flock. (Of course something could kill them all on one day, it is just less common.)
If you do build a run, be sure it is secure enough. If a predator gets in, but the chickens can not get out, the predator will probably kill every one of the chickens because they are trapped in there.