Killing him, in self-defence, is preferable to death, but is still very deep into intolerable territory. Also, if we get attacked again, then without the wards, we would quickly exhaust our mana and be defenceless... So losing Eko is not really a good plan, but, on the other hand, Eko knowing about us being a necromancer is pretty much all good. The only downside is "the more who know, the further the knowledge will spread" and the risks of a bad outcome of actually telling them. The long-term problems are almost non-existent, and it would be a huge boon to our collaborations both magical and intellectual. My inclination is to hand them a note explaining our desire to provide a unique effort to revive the soul-drained, the danger of the woods, and the threat of entire nations turned into lifeless zombie-infested deserts and then hide amidst some nearby trees. It would hopefully be more intellectual than emotive, slowing their inclination to get revenge for all the soul-sucking incidents and stop them from having an immediate target to focus upon, while still providing the opportunity to answer questions and so forth. I do fear that a necessary sacrifice would be to engage in rather extreme honesty. We could likely remain somewhat cagey about the elves, given that there safety could be endangered by such things, but really, answering all sorts of awkward and embarrassing questions promptly and openly is likely the only way to retain trust...