He/She posed it as a question worthy of discussion, s/he wasn't actually advocating murder. That's like saying, "How come bank tellers don't just rob the bank a bit every day?", a perfectly legitimate question.
> I'd probably tear up the person with the bag
"I would literally murder a random innocent" is a bit different from what you suggest.
Where on earth did you pull this dichotomy from?
Essentially, morality and right/wrong are seen as "relative". Thus, you get situations where basic, very simple morals are thrown out the window by some sort of superseding narrative. Unfortunately, we're conditioned into it every day. Long story short: "it's okay to do bad things to bad people" is glorified and romanticized. While the opposite, absolute moral of "doing bad things is bad" is ignored.
It's hard to not take this sort of discussion into the realm of politics. But, again, long story short: If we were ever to use absolute morals, we'd very quickly run into a slew of problems with most political systems every created or envisioned.
littletimmy's comment was a good occasion to apply the Principle of Charity: http://philosophy.lander.edu/oriental/charity.html. An easy plausible interpretation was that he was asking why violence doesn't happen more often—not advocating it.