As we know, in the Buddhist sense, "good and evil" are Western translations of "Life affirming" versus Life Affirming" or negative and positive influence or in the ultimate analysis, toward the correct Dharma versus against the Correct Dharma.
In this letter Nichiren congratulates a Samurai warrior for turning in his highly symbolic and rife with meaning swords (power and slaughter) from his side to the Lotus Sutra as offering to the Buddha. In this gesture the Samurai "detaches" the power and death influence of the swords from his being and converts the act of "evil" to an act of life affirming and awakened compassion ("good").