Can you use Hidden Blade to kill a unit that you've chosen as the additional cost for Cruel Patron, draw two cards, and have Cruel Patron still resolve normally?

Ruling: No, you cannot use Hidden Blade to kill the unit chosen as an additional cost for Cruel Patron. Costs are paid immediately without using the chain, so there is no opportunity for anyone to play anything targeting the unit killed as a cost.

Nuances:

• Playing a permanent does start a chain, but no player receives priority during cost payment, so you cannot react to it
• Zhonya's Hourglass works differently because it uses replacement effects, which happen instantly without requiring priority
• If Zhonya's was played before Cruel Patron (both can be played in any order as permanents without giving opponent opportunity to react), it can save the unit paid as a cost
• You cannot save a unit with Zhonya's if it's face down when you pay the cost for Cruel Patron - Zhonya's must be played before paying the cost