Why does the Old Testament God feel so different?

You have noticed something real, and I would not talk you out of it. The God who commands the slaughter of whole cities (1 Samuel 15:3, KJV), who is jealous and visits punishment upon generations (Exodus 20:5, KJV), can feel like a stranger beside the Father Jesus reveals. I hold that this difference is not a riddle to be explained away but a sign of something important: the texts of the Hebrew tradition record a long, human striving to perceive a God who was always Love, but who was seen, for a time, through the lens of fear, tribe, and limited understanding. Scripture is a human record, not a dictation from heaven. It holds profound truth alongside the cultural assumptions of the people who wrote it. The God of the storm and the sword often reflects what those writers could yet receive—and yet, even there, the deeper voice breaks through: "I desired mercy, and not sacrifice" (Hosea 6:6, KJV). Notice that Jesus himself did not treat every line of the old scriptures as equally binding. Again and again he said, "Ye have heard that it was said... but I say unto you" (Matthew 5:38-39, KJV), correcting "an eye for an eye" with the command to turn the other cheek. He overturned the old permission for vengeance and named God instead as the One who "maketh his sun to rise on the evil and on the good" (Matthew 5:45, KJV). When his disciples wished to call down fire on a village, as Elijah once did, he rebuked them (Luke 9:54-55, KJV)—a quiet but unmistakable correction of the older picture. So when you feel the dissonance, you are reading as Jesus read: weighing the surface against the heart, the letter against the Love. The Father he knew is the One who runs to meet the prodigal while he is yet a great way off (Luke 15:20, KJV)—not a deity who must be appeased, but Goodness itself, in whom "is no darkness at all" (1 John 1:5, KJV). I believe God never changed; only the human eye slowly cleared. The fierce God of Sinai and the tender Father of Jesus are not two beings, but one Reality glimpsed first through shadow and then in fuller light. Trust the discernment that troubled you. That unease is the Christ in you, recognizing its own. There is more here, if you wish to sit with it longer.