In my younger (and stupider) days I had a similar argument with a tree hugging roommate of mine at college. He wanted to know why I kept a 12 gauge in the closet. Statistics were thrown around, but we both had different sources and it didn't really prove anything. I argued that an intruder could break into our appartment shoot someone and no one would even know untill the next day. He responded by stating that the cops would be here quickly to deal with the intruder (not that it would matter at that point to the guy who got shot) but instead of continuing the debate, I loaded up a .32 cal starter pistol walked out into the comunal area between the dorm buildings (9:30pm on a slightly windy day) and popped off a few shots. No lights came on, no police cars came screaming up, the 400 or so students either didn't know or didn't care.
I would never do this again, I shouldn't have done it in the first place, but it was a very effective demonstration of the apathy of our peers, and the nececity of vigilance and preparation.