Kli Yakar (Lani'ado) #1: David was skeptical after hearing from one person, for "Ya'aseh Chafshi" (25) contradicts Mishpat ha'Melech. Therefore, he asked others around him.

Kli Yakar citing Mahari: Verse 25 says what Yisraelim said among themselves. David did not hear it.

Kli Yakar #2: Ish Yisrael said that he reviled Yisrael. David rejected this. Killing him is not removing disgrace merely from Yisrael. He reviles "Ma'archos Elokim Chayim", i.e. he blasphemes Hashem Himself! Mah Ye'aseh (automatically, from Shamayim) to the one who kills him!

Metzudas David: He wanted to discuss this, to say that he will go to fight him.

Malbim: David was astounded at the mindset of Yisrael, that the king's rewards should inspire someone to fight Galyus. Vengeance against this blasphemer is the greatest honor - is any other reward needed?!