Holiness and sin
Another reason we often have trouble with the idea that God hates sin is that we’ve anthropomorphized God. “…(Y)ou thought I was altogether like you. But I will rebuke you and accuse you to your face.” Psalm 50:21. We think that something isn’t a big deal, so we think, therefore, that God shouldn’t think so either. But God is very different from us.
To understand sin we must understand God's holiness. The most drastic example of this I can think of is the story of Uzza:
“The Ark of the Covenant was being taken to a new shrine at Jerusalem. Some obstacle, however, perhaps a bump in the road, caused the oxen to lurch, nearly upsetting the cart and putting the Ark in danger. The Bible describes the scene: "Uzzah put out his hand to the Ark of God and took hold of it, for the oxen stumbled. Then the anger of the Lord was aroused against Uzzah, and God struck him there for his error; and he died there by the Ark of God" (2 Samuel 6:6-7).”
I think most people reading this are troubled. This seems like an over-reaction on God’s part. And I can’t say that I personally, intuitively understand why Uzza’s action merited capital punishment. But that is the how the Bible describes God – and it’s important that we don’t re-make God in our own image to suit our convenience.
A somewhat helpful analogy is that of darkness and light. The Apostle John says that “God is light, and in him is no darkness (sin) at all.” 1 John 1:5. As light and dark cannot coexist, neither can God and any amount of sin. This is one of the things it means that God is holy. And this is why sins of both thought and deed, big and small, require a savior.
This also underscores that God is merciful in being absent from the earth. In the horrible state humanity is in, a holy God's arrival would be disatrous for all people who were in sin. "(God) is patient with (us), not wanting anyone to perish, but everyone to come to repentance." 2 Peter 3:9.