That's an excellent question, one you should be asking. It cannot be answered so easily and there are many different opinions and approaches.
Jesus is the "exact representation" of God's being. If we want to know what God looks like, we look at him. The absolute clearest and strongest example of God's love that we get is in the crucifixion. God, for the sake of all that he loves (including those who hate him), fights against the forces of evil by suffering and dying. He never retaliates, but prays for their forgiveness instead. The rest of the scriptures should be read through the lense of the portrayal of God we get from Jesus, in particular the crucifixion.
Cross Vision is a book which works through the Old Testament looking through the lense of the crucifixion and may be something you want to check out.
At r/cruxiformity we share articles and discuss opinions related to this view of God and the Bible as well. There is also the Christian Nonviolence Discord (https://discord.gg/NJUjDqv ) where you can find resources and ask questions related to violence and God.
UNconditional by Brian Zahnd
Cross Vision by Greg Boyd
some more years ago, this one was also incredibly important in my journey No Perfect People Allowed by John Burke
You are mis-remembering the golden calf incident. God was going to kill everyone; Moses convinced God to spare everyone, and so he did. But then Moses decides to decimate the people when he sees what's going on with his own eyes. I believe part of the point is that, when faced with the people's rebellion, Moses was not as merciful as God had been. God would have spared the people but Moses chose to decimate them.
BTW, you are also mis-remembering who was killed. He did not order the murder of all the people who had participated, there would be no one left had he done so. He ordered a form of decimation whereby each Levite killed a neighbor, a friend, and a brother.
In any case, I agree with you that it wasn't just. It just goes to show that God's ways are better; Moses should have done as God did and forgiven all of the people.
If this topic of Old Testament violence and injustice interests you, look into some of Greg Boyd's talks on YouTube; he has been speaking on this issue for the last couple of years and wrote two books on the topic: Cross Vision which is geared to a lay audience; and Crufixion of the Warrior God which is a massive two volume tome geared more towards an academic audience that needs all of the citations, footnotes, etc.