Summary: rain reduces the probability of interaction with people. Interaction with people correlates with homicides. So rain inversely correlates with homicides.
He replied with, "Meh. Criminals are lazy. They don't want the effort of earning something they can simply take, so they don't have jobs. I'm six feet and I walk fast. I'm like the strong wildebeests at the front of the herd. The criminals will wait for a weak one to fall behind. Easier."
By his logic, lazy murderers probably prefer to get their murder on without getting wet.
I'm 6'2". I have gray hair and beard, but normally am clean shaven (head and face). I walk through a somewhat tough area, and no one ever bothers me. Yes, I walk fast and with purpose.
I was having razor issues and ended up growing a (white) beard for a while. I was completely floored by the amount of crap I got from random people. It was aggressive stares, and in some cases aggressive behavior. On a single day I came close to an altercation with two people.
It was pretty obvious to me that I looked much older, so I shaved the beard and head. The next day, people were back to ignoring me.
Without an ability to test it, I'd hypothesize that rain matters more in areas where people are more likely to be outside. In areas that are more car-dominated it may matter less because you can cause trouble while staying dry.