Radak #1: We do not find that He said so. David said so in his heart.

Radak #2: Nasan said so to David, even though it is not written in Sefer Shmuel. We find many matters like this [that are written in one Sefer, but not in the other], e.g. "Nishlechah Anashim Lefaneinu" (Devarim 1:22. Parshas Shlach does not mention that Yisrael asked to send them!)

Malbim: It was after Nasan's Nevu'ah.

Malbim: They are (a) spilling blood without need in optional wars ¹ , and (b) necessary wars, of Mitzvah, or if enemies attacked, respectively.

Radak: This includes innocent blood, e.g. of Uriyah ha'Chiti. This is why it says "Lefanai". Also the Kohanim of Nov were killed due to David ? "Sabosi b'Chol Nefesh Beis Avicha" (Shmuel I, 22:22). Also, perhaps there were good Chasidim among the people that David killed in war. Even so, he was not punished for killing them, since he intended to eradicate the Resha'im, lest they spread in Yisrael, and to save himself when he was in Eretz Pelishtim. He did not leave anyone alive [lest they tell Melech Pelishtim that David kills Nochrim].

Radak: Even though David was not punished for those that he killed ¹ , this prevented from building the Beis ha'Mikdash, which is for Shalom, Kaparah of Aveiros and the crown of Tefilah. This is why we may not use iron to build the Mizbe'ach or Beis ha'Mikdash, for iron is used to make weapons that kill; it is not used much for Kelim of Shalom ² .

Gra: The Bayis will be called Menuchah ³ , therefore a man of serenity must build it.

Malbim (Shmuel II, 7:13 and Melachim I, 5:17): A man of serenity must build it at a time of serenity. David could not build it l'Shem Hashem, for he would profit through it, like Nasan said that building it will bring rest from enemies.