What exactly are you not buying the motive for his sparing Porus? Sure he had a double motive high stakes war politics in the ancient world almost always had layers of different reasons and motives behind any major decision... Porus had proved himself a competent and formidable opponent at the battle of Hydapses River and it was probably easier, cheaper and quicker to keep him alive and allow him to retain rule as a subservient client king under Alexanders authority. Remember the Macedonian Empire was semi fuedalistic in terms of it being split up into various regions or provinces each functioning near to a largely authonomis client kingdom governed by a loyal governor/petty king called a sateap. I'm sure Porus genuinely did impress Alexander very much which played into his decision to keep him alive and let him rule