Can Hidden Blade kill Ruin Runner?

Ruling: No, you cannot use Hidden Blade on an opponent's Ruin Runner because Hidden Blade requires choosing/targeting a unit, and Ruin Runner cannot be chosen by opponent's spells and skills.

Sequence:

• Hidden Blade requires a valid target to be activated
• You choose the target before putting the spell on the chain
• Ruin Runner cannot be chosen by opponent's spells, so it is not a valid target

Nuances:

• You can target your own Ruin Runner with Hidden Blade
• Hidden Blade draws cards as long as the target is still valid on resolution (even if saved by replacement effects like Sett/Zhonya's), not only when it kills the unit
• All spells recheck targeting validity on resolution