I can relate, as I am certainly someone who would probably laugh in this situation, as I also find myself laughing at the idea that people trample one another during Black Friday.
If someone were to physically injure someone during a controversial debate, I find it amusing in the sense that I am thinking "Hah... That's the way to get people to listen to your viewpoint", sarcastically.
It's not that I'm laughing at the injury. I'm laughing at the absurd immaturity of it all. Laughter is frequently used as a coping mechanism for dealing with unpleasant realities. Personally, it doesn't diminish the tragedy of what happens. It allows me to process it with a sort of mental "buffer".