It seems the early Israelites saw God's hand in everything, good and evil.For example, in 2 Samuel, Yahweh was angry with Israel, and He incited David to number them. David later recognized that He had done wrong. Then Yahweh punished Israel for this by sending a pestilence (though He had taken action to bring it about):
Again the anger of the LORD was kindled against Israel, and he incited David against them, saying, "Go, number Israel and Judah." So the king said to Joab and the commanders of the army, who were with him, "Go through all the tribes of Israel, from Dan to Beersheba, and number the people, that I may know the number of the people." But Joab said to the king, "May the LORD your God add to the people a hundred times as many as they are, while the eyes of my lord the king still see it; but why does my lord the king delight in this thing?" But the king’s word prevailed against Joab and the commanders of the army. So Joab and the commanders of the army went out from the presence of the king to number the people of Israel. 15 So the LORD sent a pestilence upon Israel from the morning until the appointed time; and there died of the people from Dan to Beersheba seventy thousand men. (2 Sam 24:1-4,15)
But as time went on, the Israelites began to attibute this evil of numbering Israel to Satan rather than to Yahweh. Then God punishing Israel seemed more just, than His punishing them for what He had enticed David to do.
Satan stood up against Israel, and incited David to number Israel. So David said to Joab and the commanders of the army, "Go, number Israel, from Beersheba to Dan, and bring me a report, that I may know their number." But Joab said, "May the LORD add to his people a hundred times as many as they are! Are they not, my lord the king, all of them my lord’s servants? Why then should my lord require this? Why should he bring guilt upon Israel?" But the king’s word prevailed against Joab. So Joab departed and went throughout all Israel, and came back to Jerusalem. And Joab gave the sum of the numbering of the people to David. In all Israel there were one million one hundred thousand men who drew the sword, and in Judah four hundred and seventy thousand who drew the sword. But he did not include Levi and Benjamin in the numbering, for the king’s command was abhorrent to Joab. But God was displeased with this thing, and he smote Israel. (1 Chron 21:1-7)
From these two accounts we can see the developing understanding of God's character. By the time Jesus appeared on the scene, it was a ubiquitous understanding among the Jews that Satan was responsible for evil and not God.
Jesus and Paul, when quoting the Old Testament never quoted the parts describing God killing people or ordering the Israelites to kill people. They were quite selective in that regard.
Here is Jesus's blessing on the merciful:
"Blessed are the merciful, for they shall obtain mercy." (Matt 5:7)
And here is the blessing of the psalmist on those who kill Babylonian babies. It is addressed to the Babylonians:
"Blessed is he who seizes and dashes your infants against the rock." (Psalm 137:8)
No mercy at all!