So wait, because you don't trust yourself enough to not stab someone why you're carrying a knife, which by the way has been a working tool forever, then all of use who carry must have murderous intent?
And the plumber, usually gets the pipes clean.
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Not everyone lives in a developed urban area. And even those who do, can still find uses for a simple pocket knife. Better to have the tool and not need it, than need it and not have it.