If your monster equipped with “Thorn of Malice” battles a monster with equal ATK while both monsters are in Attack Position, your monster is destroyed but the opponent’s is not, because the effect of “Thorn of Malice” prevents it from being destroyed.
If your “Thorn of Malice” is equipped to your opponent’s monster, it will not prevent your monsters from being destroyed in battle with the equipped monster. Your “Thorn of Malice” only prevents the opponent’s monsters from being destroyed by your equipped monster.
After the effect of "Thorn of Malice" decreases the ATK and DEF of an opponent's monster by 600, the ATK and DEF remain lowered even if "Thorn of Malice" leaves the field or is negated by an effect like "Imperial Order". The decrease to ATK and DEF lasts as long as the monster remains face-up on the field.