If you thought Black widows are just a snazzy character in a superhero franchise, you need to get out more! Except outside is where you'll find actual Black Widow spiders, with venom 15x more deadly than a rattlesnake. Actually, you'll find them inside too. Not to freak you out or anything.
This is one of those setups where you have to have some understanding for the killer. Unwanted visitors are annoying on their own, but here are thousands of them all the time. It doesn't excuse the killings, but anyone would be grumpy.