The orbital bone is very thin, so it's not hard to make it through, but if the knife doesn't go very deep you're not likely to inflict death so much as involuntary lobotomy.
Plenty of poisons can be added to knives. Obviously you have to be careful handling any knives you coat, but it's a fairly effective way if you've got the stomach for poison death. I can show you how but we'd have to do missions for poisons now that we don't have spiders anymore.