Quote from WildthoodI don't really think Malthael being much stronger than other Archangels makes much sense except for the fact that they need him as the main villain so he must be really strong. Imperius was said to be greatest warrior in all creation.
probably because now Malthael is the Aspect of Death, so that made him stronger.
Just like Tyrael, give up on one power, and gain another power. Malthael is no longer being Wisdom, and so he became death.