Are you sure it's not because of chronic exhaustion for several months in a row instead of the sex?

Essentially their entire mating cycle is what causes this. They've got a gland behind the eye that puts them into mating mode and once it starts it never turns off until they overdose on sex hormone.

Most cephalopods are voracious hunters that eat and eat to grow big and then once mating mode switches on they just focus on mating, which results in a shit ton of babies. Every step of that cycle has an extremely high mortality rate resulting in strong selection pressures for the best of every phase. When they do something, they go big.

There's a specific life history strategy called semelparity, which is what you're describing (breeding once then dying). To my understanding, this is incentivized if the chances of getting a second attempt to breed are too low, and so it becomes more evolutionarily advantageous to simply go all out on the first attempt

I read that it's so the parents don't compete for resources with their young, helping to support the young's survival

To prevent decrepit politicians who already had their chance from usurping the resources of the next generation and pulling up the ladder behind them?

You know... Octopus politicians

Reproduction is the goal. It could be as simple as giving the young a chance to out compete their r****ded parents for limited food.

Evolution doesn't care what happens to you after reproduction because you've already passed on your genes at that point

what's the point, evolutionarily, to self destruct after reproducing?

There is no point, evolution is about successful reproduction and everything else is just random chance.

If a evolutionary tweak happens that gives your off spring better chances, but your arms fall off after sex then it'll probably perpetuate.

whatever works...

Octopuses didn't get post nut clarity.