When Dusk Rose Lab checks to see if a unit was killed during the beginning phase, can I react, or does a unit have to have been killed for me to react?

To answer your question: You can react to the Dusk Rose Lab ability after it has been placed on the chain, but before it resolves.

According to the rules and FAQs regarding Dusk Rose Lab, the ability is a triggered ability that is automatically placed on the chain at the start of your Beginning Phase (alongside any Temporary triggers). Because it is on the chain, it creates a Closed State, which allows players to play [Reaction] cards.

Sequence of Events

1. Beginning Phase starts: The Temporary trigger and the Dusk Rose Lab ability are both added to the chain.
2. Chain is now CLOSED: Because the ability is on the chain, players have priority to play [Reaction] cards.
3. Reaction Window: You (or your opponent) may play [Reaction] cards now.
4. Chain Resolution: Once all reactions are played and both players pass, the chain resolves (LIFO).
5. Cost Payment: When the Dusk Rose Lab ability resolves, you then choose whether to pay the cost (killing a unit) to trigger its effect (drawing a card).

Key Clarifications

• Triggering vs. Resolving: The ability is placed on the chain automatically at the start of the phase. You do not choose whether to put it on the chain; you only choose whether to pay the cost (killing a unit) during its resolution.
• Reaction Timing: Because the ability is on the chain, you do not need to wait for a unit to be killed to react. The ability itself is a chain item that you can respond to immediately after it is placed on the chain.
• Official Support: This interaction is explicitly detailed in RiftJudge FAQ #9428 and #9473, which confirm that the Temporary trigger and the Dusk Rose Lab ability are added to the chain at the start of the Beginning Phase, allowing for a reaction window before the cost is paid and the effect resolves.

In summary, you do not need to wait for a unit to be killed to react; the presence of the Dusk Rose Lab ability on the chain is sufficient to open a reaction window.