Can you cull the weak a ruin runner?

Yes, you can play Cull the Weak even if your opponent controls a Ruin Runner.

Here is the breakdown of why this interaction works:

• Cull the Weak does not target: Cull the Weak does not target or choose any units when it is played. The instruction to kill a unit is performed by each player during the resolution of the spell.
• Ruin Runner's protection: Ruin Runner has the ability: "I can't be chosen by enemy spells and abilities." Because Cull the Weak does not "choose" or "target" units, it does not violate the protection ability of the Ruin Runner.
• Resolution: When Cull the Weak resolves, each player must kill one of their own units. If your opponent has a Ruin Runner and no other units, they will be forced to choose the Ruin Runner to satisfy the spell's effect, as the spell's effect is mandatory and does not rely on targeting.