It was an expressway. There were no lights other than cars. You're not wrong, had a human sprinted at 20mph across the expressway in the dark, I'd have hit them, too. That being said, you're not supposed to swerve and I had less than a second to react from when I saw it. It was getting hit and there was nothing I could've done.
I am neither blaming you nor critiquing your actions. In fact I agree that we should not swerve. I was just making an observation that driving slightly slower in low visibility might help by giving you more time to notice an obstruction and brake while provide also providing more time for the obstruction to react and clear the road. At least very least, people might slow down enough so that the crash is no longer fatal to the person or animal being crashed into
I haven't hit a deer, not even come close since they aren't a problem in my country. You are most probably right and i have seen videos of deer just jumping onto the road at the last second which causes an unavoidable accident. My viewpoint is that when you hit a creature(animal or human) at 80mph, they are most certainly dead. If you hit them at 60, they might survive but be gravely wounded. If are able to react and slow down before contact to about 30, they will be hurt but at least they have a much better chance of the survival. Somehow going at same speeds during the day and during the night seems very risky