Were the adultery and murder committed by David any less serious than the sins of Ahithophel? Why did God’s treatment of the two men differ?

No—David's adultery and murder were in no way less serious; both men were sinners, David with a lascivious trend in his sin nature and Ahithophel with a legalistic one. The difference was not the size of the sin but the response to it: David understood grace. He had been in reversionism, but he accepted God's correction and was restored through confession, so he knew both how to receive grace from God and how to extend it to others. Ahithophel refused the discipline God faithfully sets before all of us when we sin, and so his reversion into legalism ran all the way to death, while David's reversion into lasciviousness did not.