If you pick discretely and not flick in another's path tis no sin at all.
However if you pick at the edge of a subway rail, flick on the person next to you, and in their excitement they fall on the third rail or get creamed by a train - I'm sure there was some sinning in that somewhere, even if the world is better off without them.
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“Love takes off the masks that we fear we cannot live without and know we cannot live within. I use the word "love" here not merely in the personal sense but as a state of being, or a state of grace - not in the infantile American sense of being made happy but in the tough and universal sense of quest and daring and growth.” - James Baldwin