Yes kings Shaul and David were very righteous Kings and killed thousands of wicked people from other nations BUT WHO DO YOU THINK ACTUALLY SENT THEM????? They were direct messengers of Hashem chosen by Hashem to be the one’s to kill these thousands of wicked people.
Just like every single person has a mission to complete in this temporary physical world before being able to enter the true olam haemes of the world above and serve Hashem openly with complete spirituality.
May we all come to the level of realizing that everything going on in the world-from tiny bugs to president Biden-is actually the doing of Hashem directly as the famous saying we all know says “Hakol biyidei shomayim chutz miyiras shomayim” which literally means that everything going on in the world is in the hands of Hashem except for fear of Hashem.