In John 8:59 (and in John 12:36) the voice of the verb κρύπτω is passive which I understand to mean that the subject (Jesus) is receiving the action of the verb as opposed to doing the action of the verb. Why then is ἑκρύβη in most translations translated as Jesus hiding himself in John 8:59 and John 12:36? Is this a valid translation?
Indeed, you can parse ἐκρύβη as a form of κρύπτω, the aorist passive indicative. However, the verb κρύπτω admits a translation in the passive voice which evokes to the middle voice. In other words, according to the Liddell-Scott-Jones Greek-English Lexicon (emphasis added):
[...] —Pass., hide oneself, lie hidden, οὐρανῷ κρύπτεται E.Hel.606; δαλὸς κρύπτεται ἐς σποδιάν Id.Cyc.615 (lyr.); ὑφ’ εἵματος κρυφείς S.Aj.1145 [...]
Compare with Luke 19:42 and Hebrews 11:23, where ἐκρύβη has a passive sense (to be hidden).
As you can infer, the passive forms of κρύπτω can express either middle or passive meanings and it can only be distinguished by the context.
Q: Did Jesus hide himself or was he hidden in John 8:59?
A: Both actually
Jesus is making himself invisible so that they can’t stone him. It’s not yet his hour so he needs to slip through and away from the crowd. While he’s the one performing the action, he’s also the subject being made invisible. http://biblehub.com/john/8-59.htm
Gill Exposition: “But Jesus hid himself, not in any corner of the temple, or behind a pillar; but he withdrew himself from them directly, and made himself invisible to them…”