Randall's comics are not always intended to be humourous. He likes to point out loopholes & inconsistencies in legal / social rules. A society can have all sorts of draconian rules in place designed to prevent a tiny proportion of the population doing something nasty which inconvenience almost everyone, yet there are all sorts of scenarios like the one in this comic that can have dire consequences that the rules can't prevent (although it's of course quite possible to prosecute the perpetrator after the event).
As regards the "funny/not funny" discussion, I think this is the most important point. xkcd is frequently funny, but not always, and I don't think there's a contract out there anywhere that says that it has to be. I can almost guarantee that every single person on the planet has had a momentary flash of wanting to hurt someone when frustrated or angered by something. For me, it's the unpreventable thought that pops in my head occasionally about how easy it would be to bump somebody into oncoming traffic when I pass them on the sidewalk... and I've never done it, and I never will, and it makes me cringe at myself to admit it... I think this comic made me laugh because laughing relieves the tension that you feel all day long at dealing with people, and then you shrug and smile and help out the next person in line.
Randall seems to use xkcd to express himself and frequently that makes other people laugh. And while I imagine he's pleased when it does, I can't imagine he loses much sleep when someone complains that something that may or may not have been intended to be actually humorous makes other people gasp! shocked! appalled!. And from the sounds of things, it's been a trying time in life lately. Cut the guy some slack when he vents his frustration through his artwork. Cripes. For those who think this isn't funny, never go hang around with a group of doctors, or firefighters, or police officers, or any other service profession, when they're blowing off steam.
(And, since this is my first time posting- thanks, Randall, you've been making me laugh for years. Now GOOMH!)