Why did God kill 70,000 men because of David's census?

Because the census was not innocent bookkeeping. David counted his soldiers because he had stopped trusting God to defend Israel, and as king his breach of faith brought guilt on the whole nation he represented. A census by itself is nothing. Every country takes one. You want to know how many people you have, and in the ancient world that meant chiefly how many fighting men you have, because you may have to defend your territory. That is not a sin. What made this one a sin was why David did it. By 2 Samuel 24 David is an old man, and something has slipped. Having a strong faith was easier when he was young. We saw the same thing happen to Solomon, who faltered in old age, and it happens to plenty of men. David has amassed a great territory and he wants to hold it, and so he sends Joab out to count the soldiers. Why? Because he is no longer depending on God to preserve what God gave him. He is depending on the arithmetic. If a man can say to himself, it is all right, I have half a million soldiers, and never once think of the God who has been carrying him this whole time, that man has quietly changed who he trusts. That is the malice in the act: David is showing contempt for the Lord by not trusting Him to deliver Israel regardless of the number of fighting men. It is a serious sin and a breach of faith, and it is the same species of sin as Abraham laughing when God promised him a son. Joab, of all people, saw it. He protested to the king: may the Lord add to His people a hundred times as many as they are, but why does my lord require this? Why should he bring guilt upon Israel? Joab was not squeamish about killing. He understood the theology. The king's word prevailed anyway, and Joab went and counted. Then comes your question, and it is the right one to ask: why did the people die? Because a king is not a private man. He represents the nation before God, and Joab had said out loud what was at stake, that David would bring guilt upon Israel. When a leader sins in his office, the people under him bear the consequences; that principle runs through the whole of Scripture and it runs through the Church too. And notice the exact shape of the punishment. God struck the very soldiers David had numbered. David had put his trust in a number. God took away the number. The prophet Gad brought David three choices: famine, flight before his enemies, or three days of the sword of the Lord. Two of those were punishments at the hands of men; the third was from God directly. David chose the third, and he chose it deliberately, reasoning that it is better to fall into the hands of God than into the hands of men, because God punishes and also has mercy and may lessen the sentence. He was right. The angel killed seventy thousand, and then God Himself was grieved at the destruction and commanded the angel to stop before Jerusalem. What David does next is worth more than the whole debate about the numbers. Protestants who read this chapter never notice it. In the middle of the punishment David reasons that if he offers a sacrifice, God may relent of the rest of it. So he buys the threshing floor and the oxen with his own money, refusing to offer to the Lord what cost him nothing, and he makes the sacrifice, and the plague stops. There it is: the sin was already forgiven, David had already confessed, and yet there was temporal punishment still to be worked off, and a voluntary act of sacrifice made in faith took some of it away. That is the biblical foundation of what the Church calls temporal punishment and indulgences, and it is sitting in 2 Samuel 24 in plain sight. We can offer our money, our bodies, our time. God sees the heart and the sacrifice and remits some or all of the temporal punishment. One more thing, since it always comes up. 2 Samuel 24:1 says the Lord moved David to number Israel, and 1 Chronicles 21:1 says Satan stood up against Israel and provoked David to number Israel. That is not a contradiction; it is the standard biblical way of describing a permitted temptation. Satan is the immediate agent; God permits it and stands behind the whole event as its sovereign. Scripture does the same thing with Job, and with Pharaoh's hardened heart. The two verses are two vantage points on one act, not two irreconcilable accounts.