My favorite rabbit trail to drag the "life begins at conception! The moment the sperm meets the egg, a new soul is created!" crowd down is to start theorizing at length about identical twins, naturally occurring clones that divide... after fertilization. It always takes them off guard and completely derails the rant.
Does each twin have half a soul? Is there one full-souled twin and one soulless twin? Did you know that in a lab you can cut a freshly fertilized embryo in up to eight pieces and they will all develop completely? Would this produce seven extra lab-created soul or one lab-created Lord Voldemort split seven ways?
And then we move on to the opposite problem: what about naturally occurring human chimeras, a pair of fraternal twins that collide and merge into one organism with the DNA of both... after fertilization. Does a chimera have two souls? Two half-souls that don't match? Does one soul kill the other? Both sets of DNA survive, so what determines which soul lives and which dies? Does that make the surviving soul a murderer before it even develops a heartbeat, let alone awareness?
And given that both identical twins and human chimeras are psychologically indistinguishable from anyone else, what does a soul actually do? What are the theological implications, if your theory of soul-at-fertilization requires drastic soul weirdness when confronted with biological reality, but that soul weirdness ends up producing... no effect whatsoever?