Assume you explain to your friend that you are in favor of the death penalty, and your reason is that there are too many people in the world anyway. What form of fallacy of relevance would this be?

Red-herring.

Explanation:

Fallacy is elucidated as the flaw or error in reasoning while constructing an argument that affects the validity and credibility of the argument. Red-herring fallacy is demonstrated as the logical fallacy in which a misleading or deceptive narrative element is introduced in an argument that primarily intends to distract the readers and leads them to false conclusions. Such fallacy is often employed in suspense or thriller narratives in order to enhance the readers' interest and curiosity by misleading them.

In the given example, 'red-herring' fallacy is employed which is reflected by the introduction of the insignificant/irrelevant detail like 'there are too many people in the world anyway' that invalidates the efficacy of the argument ('favor of the death penalty') and distracts the readers from the original claim. Therefore, 'red-herring' fallacy is the answer.