If Ruined Rex dies, do I need to use the deathknell, or can I opt not to?

According to the official Riftbound rules and RiftJudge FAQ #5520, you cannot opt out of using a Deathknell ability.

Deathknell triggers are mandatory. As established in Rule 808.1, Deathknell is a triggered ability that occurs automatically when the permanent is killed and sent to the trash. Because Ruined Rex's text does not include the word "may" (e.g., "you may deal 4"), the effect must occur.

If there is a valid target for the effect, you must resolve it. If there are no valid targets on the board, the ability will still trigger and enter the chain as a pending item, but it will simply do nothing when it resolves.