Praying Mantises only sometimes eat their partner's head, in one study it was 20% of the time, in another it was 1 in 70. It's not that different from other species of predatory loner bugs who will often end up cannibalistic if they encounter another member of their species. Even octopuses run into the trouble where sometimes their mate may just prefer to nosh on them rather than ho through with the mating.
But then people act like it's a whole mantis obsession, like that's all of what they're about. It's bad bug science, and I'm disappointed. They better not go to a wolfgirl shop and wind up with everything being about that long-debunked alpha/beta/omega theory.