Why do female black widow spiders kill their males after mating?

Answer 1

To stop the snoring before it starts

Answer 2

The short answer is, they don't, at least usually.
An aquaintance of mine who reared black widows for his research told me he had watched
about 500 matings of black widows and had never seen the female kill the male.

Any spider of either sex may kill its prospective
mate under some circumstances. One is that they
are disturbed or in unfavorable conditions for
some reason. Another is that the female has
already mated with another male and is no longer interested in anything but food. I have
seen this latter happen with a pair of jumping
spiders I had alive. When they were first put
together the male began courting immediately
and in a few minutes they mated. About a week
later I put the same two together again and the
female immediately attacked the male.

Answer 3

Sexual cannibalism is an extreme form of parental investment... by being eaten by the female the male is providing her with a lot of energy, increasing the chances that she will survive and that their eggs will hatch, which maximizes his reproductive fitness. But, not all black widow males are eaten, many of them have developed adaptations to protect themselves from the females.

Answer 4

Food source...

Answer 5

because they are smart!