Regarding etorphine (I think we had this discussion a few months back).
It's just a potent opiod. It's not categorically fatal to humans. In fact it was trialed for the treatment of cancer pain in the 70s [1]. There's case reports in the literature of intentional overdose with respiratory depression responsive to as little as 0.8mg narcan [2].
It just doesn't have a current FDA accepted use in humans, but is used in vetinary medicine, where it's useful for sedating large animals, due to it's high potency (1,000 - 80,000 times the potency of morphine, dependent upon the species considered)[3].
So, like any opiod, if you give too much, apnea will ensue, which, if untreated, will be fatal. It sounds like the agent probably doesn't offer benefits over other currently available opiods. It's the old Paracelsus, only the dose separates the poison from the cure.
[1] Blane GF, Robbie DS.Trial of etorphine hydrochloride(M99 Reckitt) in carcinoma pain: preliminary report. Br J Pharmacol. 1970 May;39(1):252P-253P.
[2] Sterken J, Troubleyn J, Gasthuys F, Maes V, Diltoer M, Verborgh C.Intentional overdose of Large Animal Immobilon.Eur J Emerg Med. 2004 Oct;11(5):298-301.
[3]Lancet. 1977 Jul 23;2(8030):178.
Immobilon: curiously strong.