Since the passion is below 16, you're free to take whatever action you wish. It is once your traits rise to 16 or higher that they can start dictating your answers. However, your actions may earn you a check against the trait or passion in question during the winter phase.
But to play out your example, if you had a passion Hate (Saxons) of 16 or higher, and wanted to be merciful, you would need to roll a check against the passion to avoid killing him. If the passion wins, you kill him even though you might not want to.
Either way, killing him might earn you a check against Cruel.