Sforno on Deuteronomy 20:5/ספורנו על דברים כ׳:ה׳

1פן ימות במלחמה ואיש אחר יחנכנו, perhaps the man in question had been guilty of a sin on account of which he would be punished by death or one of the punishments mentioned in the Tochahach, the lengthy warnings of Moses in chapter 28. His death would undermine the morale of his brothers in arms.