I play Ranger Spikes into the Staging Area.

A bad guy pops out, getting Ranger Spiked.

I kill the bad guy who is Ranger Spiked.

Ranger Spikes go to my Discard Pile, or go back to the Stagine Area? I would normally assume my Discard Pile, but it does say "If unattached, attach Ranger Spikes to the next eligible enemy that enters the staging area."

It goes to your discard pile. The other text on the card just makes sure that the spikes only attach to an enemy if they are unatached and not every time a new enemy comes of the deck. If you lose the the second sentence on the card, the spikes would attach again and again and again, always changing enemy.