When playing Cull the Weak, which player has to choose first?

When playing Cull the Weak, the choice of which unit to kill is made by each player during the resolution of the spell.

According to the official rulings for Cull the Weak:

• No Targeting: The spell does not target units when it is played. Because it does not target, no choices are made when the spell is finalized onto the chain.
• Resolution Choice: The instruction "Each player kills one of their units" is performed during the resolution of the spell.

Regarding the order of choices: In Riftbound, when a spell or ability requires multiple players to make a choice simultaneously during resolution, players perform the action of choosing and killing their unit as part of the spell's resolution process. If a situation arises where the order of resolution matters, the game follows the standard turn order or the active player's priority to resolve the effects.

If you have no units, you simply do not kill one, but your opponent must still kill one of theirs if they have any, following the "do as much as you can" principle (Rule 356.3.e.11).

Note: I could not fully verify the specific ruling regarding the order of choices against the retrieved rules.