Because it shows she respects all life no matter how small. Its more along the lines of "it didn't hurt you so why hurt it?" and the fact that even if it did hurt her, it is a life created by a higher being and therefore, not her place to take. That is why she is told fighting is a last resort and it ends in victory for nobody because someone is caused harm by another person instead of reaping what they sow from the higher power. Also, it is implied the monks are buddhist and so they believe in reincarnation so to take it further, that little bug may contain someone's soul so its like murder to kill it