It's not as common as it's made out to be, but it does occur sometimes.
As for the spiders, I'm guessing you're talking about 'black widows'? It's a similar case with them as well; misunderstood by misogynist male scientists (jk). The male half of the species is just as capable of such 'evil', not to mention-- they're 'ageist' too

:
"
But in the species Micaria sociabilis, males are more likely to eat the females than be eaten, a new study found. Meanwhile,
age might be a dooming factor for the female Micaria sociabilis. There was a peak in reverse sexual cannibalism when males from the summer generation encountered older females from the spring generation, the researchers found. What's more,
not even virginity or big body size, often considered signs of mating quality for spiders, could save the older females from male cannibalization."
https://www.livescience.com/29345-male-black-widows-sexual-cannibalism.html