What is the meaning of "va'Yach Lev David Oso"?
Targum Yonasan: He was pained in his heart. Radak - i.e. he was concerned for his sin, lest he and Yisrael will be punished for it. He feared in his heart and regretted, and asked Hashem not to punish for his sin.
Why does it say "Acharei Chen Safar"?
Radak: This is like "Asher Safar." There are many verses like this.
Malbim #1: He regretted that he counted them in the way he did (refer to 24:2:1:1). Had he been Poked them, like Sha'ul, there would not be any sin.
Malbim #2: In Divrei ha'Yamim I, 21:7 it says that Hashem struck Yisrael. [After, i.e.] due to this, David regretted.
Why did David mention Chet and Avon?
Malbim: Chet refers to the action. Avon refers to errant intellect. Both applied here. "Chatasi
