''Because it's impossible that a person (or a personal God) could act in different ways at different times?''
Of course it's possible....that's why we call a malicious act out of character, as a case of contradictory behaviour. To act vindictively inn response to a transgression is contra to forgiving a transgression. You should know what the new testament says about the value of forgiveness. And of course, God is said to be eternal and unchanging...which also contradicts your rationale.
