Can you play Cull the Weak in combat if it's not an action or reaction?

No, you cannot play Cull the Weak during combat.

According to the RiftJudge FAQ (ID 8041), spells that are neither [Action] nor [Reaction] (often referred to as "Main" timing spells) can only be played during your Main Phase, out of a showdown, when the chain is empty.

Why you cannot play it in combat:

1. Timing Restriction: Combat is a "Showdown State" (Rule 343.1). During a Showdown State, cards that are not [Action] or [Reaction] cannot be played.
2. Missing Keywords: Cull the Weak does not have the [Action] or [Reaction] keyword, which are the only permissions that allow a card to be played during a showdown or when the chain is not empty.
3. Spell Category: Because it lacks these keywords, it is restricted to your Main Phase when the board state is "Neutral" (no combat/showdown) and the chain is empty.

While you do not need to control a unit to play Cull the Weak (as it does not target and follows the "do as much as you can" principle), you must always respect the timing permissions of the card itself.