If the utility monster is so monstrously sad, why would it be asking not to be killed? Usually, a decent rule of thumb is that if someone doesn't want to die there's a good chance their lives are somewhat worth living.

The converse perspective implies that we should either (1) be spawning as many babies as possible, as fast as possible, or (2) anyone who disagrees with 1 should go on a murder spree, or at best consider such murder sprees ethically unimportant.

This conclusion is technically incorrect. For new babies, you don't know in advance whether their lives will be worth living. Even if you go with positive expected value (and no negative externalities), you can still have better alternatives, e.g. do science now that makes many more and much better lives much later; "as fast as possible" is logically unnecessary.

Also, killing sprees have side-effects on society that omissions of reproduction don't have, e.g. already-born people will take costly measures not to be killed (etc...)