eli5- Why do insects and arachnids like a praying mantis and spiders kill the one they mate with?
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This is known as [sexual cannibalism](https://en.wikipedia.org/wiki/Sexual_cannibalism) (which is a great band name).
Many hypotheses exist as to why it evolved. Two basic ones are that either:
1. The females are simply hungry and if the closest ready food source happens to be the male they just mated with, so be it.
2. The males are “willingly” (it’s hard to ascribe agency to instinctual behavior) giving their lives to provide nutrition to the female, which increases their offspring’s chances of surviving. After all, in lots of species the males simply die after mating, so why let those calories go to waste?
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