Killing is a vestigial instinct in mankind. Our sentience has moved us past it. Most of the time when people kill, it hints towards varying levels or forms of psychosis. If not, it is usually a crime of passion or in some form of self-defence. Usually though, we aren't killers. If you could talk to a spider and asked it if they thought that was okay, they would say yes because she was hungry. That is how they operate. They don't have the same level of sentience as us that allows us to question our morality about such incidents. While we wouldn't consume our mates, that is because 1.) Humans are not cannibalistic in nature. While there some noted examples, it should b known that they are oddities. 2.) We have feelings such as love and friendship. We form stronger and more emotional bonds than other animals do. We actually consider the judgement of our peers, whereas animals try just to survive. They might bow out of the way of the alpha male, but that is typically out of fear as opposed to respect which would be a more logical reason for a human to act the same way towards someone who posesses dominance over them such as their boss, or a parent. While fear of punishment factors into this, I find that it is typically to weak of an emotion in many cases unless the individual is noted for causing significant phyiscal or emotional trauma.
I suppose you may have a point. Of course, petty is too subjective anyway. I think the main point is, if we found out it was happening somewhere in the world, the indignation would resonate around the world and the UN and NATO would probably step in. It's hard to say that the fact that it's instinctual makes it less "petty", since for a lot of killers in our species, it's instinctual as well, but we still view them as reprobates.
Well, I don't know if I would necesarrily establish that as a "petty" reason. Like I said, spiders are cannibalistic, and females often have a difficult time establishing whether or not a particular spider is of the same species as it. If they are hungry enough, they will act instinctually like all animals do and consume the easy prey in front of it. Unlike humans, they mate simply to create offspring, so morality is entirely out of the question. Spiders might not be the best example to illustrate such a point.
Black Widows actually eat their mates because all spiders are cannibalistic in nature. And while people mainly know Black Widows for doing it, it is actually ALL spiders. When they begin the courting process, the female enters an altered state of mind. After the deed is done, she returns to her normal mindset which is to eat anything smaller than her(and males are typically the smaller spiders). So, the males tend to leave and find another female as soon as they are done passing off their genes. If the female is hungry enough, she might just eat him instead of reproducing, however.