It's a matter of numbers.
If a Borg walks around the corner, you kill it. Then another one comes, you kill it. Another one, you reset the harmonics or whatnot and kill it. What if another one comes? A whole cube? Do you refrain from killing it because you don't want to kill too many?
Picard was hiding
behind morals, in the sense that even if he knew it was bad, he could do it as a sacrifice of himself ( not his life but of his conscience) and rid the galaxy of the Borg.
He could go home and cry about it while the rest of the galaxy builds statues and renames whole planets in his honour.