In his previous killings, God killed indiscriminately. He drowned everyone and everything in the flood (1), smashed people with burning stones at Sodom and Gomorrah (3), and killed every Egyptian firstborn child and animal just for the heck of it (9). So I guess we should be used to this sort of thing by now.
But in this killing, God forces 3000 friends and family members to kill each other. That seems kind of nasty even for a very nasty god.
Here are the gory details.
Moses was up on Mount Sinai getting the Ten Commandments from God. Since he'd been gone so long (he'd been up there for 40 days) the people began to wonder if he'd ever come back. So they asked Aaron to make some other gods for them. Aaron thought that was a pretty good idea, so he
Said unto them, Break off the golden earrings, which are in the ears of your wives, of your sons, and of your daughters, and bring them unto me ... And he ... made it a molten calf. Exodus 32:2-4
You might think that a bunch of runaway slaves wouldn't have much gold. But God told them to steal whatever jewelry they could find from the Egyptians.
The children of Israel did according to the word of Moses; and they borrowed of the Egyptians jewels of silver, and jewels of gold, and raiment.... And they spoiled the Egyptians. 32:35-36
I guess God wanted them to have enough gold to make a golden calf. It was all part of his plan.
So the people gave Aaron their stolen gold and Aaron made a golden calf.
Now making a golden calf out of a pile of ear rings and a campfire might seem hard to you. But Aaron just threw them all onto a fire and out came a golden calf. Really.
I [Aaron] said unto them, Whosoever hath any gold, let them break it off. So they gave it me: then I cast it into the fire, and there came out this calf. 32:24
It was a miracle. God (or Satan) made the golden calf when Aaron threw the jewelry on the fire.
In any case, when Moses came down from the mountain, he saw the people dancing naked ("for Aaron had made them naked") around the golden calf.
Moses saw that the people were naked; (for Aaron had made them naked.) 32:25
So he smashed the stone tablets, burned the golden calf, ground it into a powder, sprinkled it on water, and then forced everyone (all 3 million of them) to drink it.
As soon as he came nigh unto the camp, that he saw the calf, and the dancing: and Moses' anger waxed hot, and he cast the tables out of his hands, and brake them beneath the mount. And he took the calf which they had made, and burnt it in the fire, and ground it to powder, and strawed it upon the water, and made the children of Israel drink of it. 32:19-20
But Moses was just getting started. Here's what he said next:
Who is on the Lord's side? .... Thus saith the LORD God of Israel, Put every man his sword by his side, and go in and out from gate to gate throughout the camp, and slay every man his brother, and every man his companion, and every man his neighbour ... and there fell of the people that day about three thousand men. 32:26-28
So those on God's side went out and killed 3000 of their friends, neighbors, and family members.
But God still wasn't satisfied. When he first found out about the golden calf and the naked dancing he wanted to kill everyone and start over with a new batch of people.
The LORD said unto Moses, I have seen this people, and, behold, it is a stiffnecked people: Now therefore let me alone, that my wrath may wax hot against them, and that I may consume them: and I will make of thee a great nation. 32:9-10
But Moses talked him out of it, saying, "What would the Egyptians say?"
Moses besought the LORD his God and said ... should the Egyptians speak, and say, For mischief did he bring them out, to slay them in the mountains, and to consume them from the face of the earth? Turn from thy fierce wrath, and repent of this evil against thy people. 32:11-12
Imagine that. Moses is a nicer guy than God. (See the 28th killing for the implication here.)