梵網經菩薩戒本疏 1.54

1.54

第二、正制者，「不得故殺」者，謂無害義，故云「不得殺」也。然錯誤無犯，故復云「故」。

Second, the explicit proscription: 'must not intentionally kill' means the absence of harm, hence it says 'must not kill.' However, in mistake and error there is no offense, hence it further says 'intentionally.'