Radak #1: There was anger also due to Avshalom's rebellion. Even though it was a punishment for David - he had to flee, and his wives were raped, and also 20,000 Yisraelim died in one day, Hashem was angry due to covert sins.

Radak #2: 'Was added' refers to three years of famine.

Radak: If there was evil in Yisrael, David would eradicate it! Perhaps people sinned covertly. Therefore, the punishment came via a reason (counting Yisrael).

Malbim citing Mahari: It was because they rebelled against Malchus Beis David and followed Sheva ben Bichri, to cast off the yoke of Malchus Beis David and Malchus Shamayim. This itself caused David to count them, for he did not trust that they will be loyal to fight when needed (refer to 24:13:6:1).

Radak (from Tanchuma Ki Sisa 9): Even indirectly (e.g. via Shekalim or sheep) is permitted only for a need. Whenever they are counted without need, their number is reduced.

Ramban (Bamidbar 1:2): One may count only those above 20; then, "v'Lo Yihyeh Vahem Negef" (Shemos 30:12). David erred to think that giving Shekalim for Kofer Nefesh (mentioned just before this in that verse) averts the plague. (Really, to avert the plague one must count only from above 20, like the verse after that discusses.) Yo'av suspected this, and wanted to count only the adults. This is why he began counting, but did not finish (Divrei ha'Yamim I, 27:24). ¹

Malbim: Rashi and the Ramban (Shemos 30:12) say that David's sin was to count directly, without Shekalim. Mahari disagrees. He says that they counted Yisrael and the Leviyim directly in the Midbar and in Arvos Mo'av, and also Sha'ul counted twice without Shekalim. He, Ralbag and Akeidas Yitzchak (Ki Sisa) hold that the sin was to trust in the number of Yisrael. I will say merely that the Torah testified that counting can cause a plague - "v'Lo Yihyeh Vahem Negef bi'Fkod Osam" (Shemos 30:12). There are three possible reasons. (a) Ayin ha'Ra - from an envious person, harmful vapors leave his eye with ability to kill people vulnerable to this, just like an Ef'eh ² can kill through looking. (b) Blessing comes only on something hidden from the eye. The existence and increase of Yisrael opposes the Ma'arachah (arrangement of stars). Hashem removed them from the Ma'arachah's influence, and put them under His Hashgachah of hidden miracles. Hashem does not openly override the Ma'arachah. When their number is known, the miracle will be revealed; Hashem will not do so ³ . (c) As long as Yisrael are united like one man, the merit of the Rabim is great. When they are counted, each man is considered by himself; his sins are sought, and a plague results.

Radak: No. Had He done so, David would not say "I sinned" (verse 10)! Rather, He put the idea in David's heart ¹ , and David told one of his officers to do so. David did not know that it is from Hashem. This is what it says in Divrei ha'Yamim I, 21:1 that the Satan enticed him.

Malbim: There are four expressions of counting - Meneh, u'Fikdu, [I will know] Mispar [ha'Am - verse 2], and (in Parshas Naso) Naso. The verb Saper refers to the count itself. Pakad refers to the total number, and hints to the purpose of counting, e.g. to know the number of soldiers, who will go to war and how many will go. "Hashem Tzevakos Mefaked Tzeva Milchamah" (Yeshayah 13:4). ¹ Anything counted one after the other, it says only Saper, but not Pakad, e.g. "Asaprem mi'Chol Yirbeyun" (Tehilim 139:18), "Tze'adai Tispor", "Tispor Yerachim Temal'enah" (Iyov 14:16, 39:2), "v'Hikahu Kedei Rish'aso b'Mispar" (Devarim 25:2). Mene comes even if the number was already known, and he counts to know if anything is missing - "Od Ta'avornah ha'Tzon Al Yedei Moneh" (Yirmeyah 33:13), "Limnos Yameinu Ken Hoda" (Tehilim 90:12), "v'Chisaron Lo Yuchal Lehimnos" (Koheles 1:15), "Moneh Mishpar la'Kochavim" (Tehilim 147:4), i.e. their number should never diminish. "Im Yuchal Ish Limnos Es Afar ha'Aretz" (Bereishis 13:16) - number does not apply to earth, for if one takes fine dust from earth, it remains intact like it was. So Yisrael, when there is a Chisaron, it is immediately filled ² - "Mi Manah Afar Yakov u'Mispar Es Rova Yisrael" (Bamidbar 23:10); Minuy is said about Afar. Naso applies to counting that has importance, e.g. Giborim in the army. Bamidbar Rabah 4 says so. This is why in Chumash Bamidbar, it says only Naso; the count was for importance, and not to know is anything was lacking ³ . Its purpose was to know the number, so it says [also] Pekod. Also Sha'ul "va'Yifkedem ba'Tela'im" (Shmuel I, 15:4). In Bamidbar (1:2), it was "b'Mispar Shemos." He did not count heads; then, Ayin ha'Ra applies. Rather, he counted names. Everyone wrote his names; via counting the names, they knew the number. Also according to the third reason, that people are singled out, [Ayin ha'Ra did not apply], for they gathered the total sum. Also the second reason (Brachah rests only on what is hidden, this did not apply), for he counted only the soldiers, and those below 20 were not counted; there was room for a hidden miracle. However, in Yo'av's count it uses three expressions to reveal David's intent, and how he sinned. Refer to 24:13:6:1.

Radak: He put in his heart to count them.

Malbim: It was as if Hashem aroused hatred between David and Yisrael, that David suspected lest they not be loyal to come to fight when he calls, so he saw a need to count them (refer to 24:13:6:1).

Malbim: David was concerned lest the nation not be loyal to come to fight when he calls (refer to 24:13:6:1). His primary concern was for Yisrael, who followed Sheva ben Bichri. He counted Yehudah merely along with Yisrael.