Even just in the Gospels, Jesus sometimes deals harshly with sinners (n.b. the "cleansing of the temple"). The OT spans a much greater period of time, and has many more dealings with evil gentiles. The argument being made is like saying that the overtly sadistic torturer Gul Madred and the loving, doting, devoted father Gul Madred are different people. No; they're the same person, behaving differently in different situations.