I'm finding throwing knives in JA2 to be very unreliable.
I'm behind the target, I'm pretty sure I'm undetected yet it still doesn't instant kill the target. Sometimes it works, sometimes it doesn't.
I've been using bloody throwing knives, is that why? On the wiki it said there's no difference.

It helps a lot if the merc has throwing as a specialty. I've definitely downed enemies with two knife throws.
If the enemy is heavily armoured, then knife throwing might become a bit futile.

It's probably a better idea to get some suppressed guns, although these will also be less effective against later enemies, since they'll be wearing better armour.