Sex And Violence Go Together For Many Invertebrates

For humans, we often say that sex is a sign of commitment. But, octopi, insects and spiders are often really committed.

It’s not often that copulating octopuses eat one another—but it’s not uncommon either. “There just isn’t any way to measure how frequently it happens,” says David Scheel, a ecologist who studies octopuses at Alaska Pacific University. There are over 300 species of octopuses, and most are hard to study because they reclusively lurk in the depths of the ocean. Of these, a handful—including the giant pacific octopus—have been known to brutally murder and eat each other after sex. In 2014, researchers described (paywall) an instance in which a female octopus had sex with a male for 15 minutes, and then effectively strangled him with three of her tentacles by blocking his gills. 

But octopuses aren’t the only ones who kill their sex partners. Female praying mantises often kill their mates, especially if they’re hungry, and within certain species of spiders, the males will actually offer themselves as a meal for their newly-impregnated partners. 

Despite the ferocity of mating in the animal kingdom, romance is not dead: sexual cannibalism can be something of a gift from the male to the female in many cases. Female wolf spiders and tarantulas, who often eat males pre-intercourse, produce 30% more eggs than those who don’t when they finally get around to mating. And in the mantis’ case, the death of one male often means the survival of the reproducing female.

From here.

In the same vein, some fish, like salmon, while they aren't necessarily eaten by their partners, do make an epic journey to spawn and die when they finish.

I can't think of any terrestrial vertebrates that follow either pattern but would be happy to be corrected in the comments.