1)
How do we see David's love for Pikudei Hashem?
1.
Radak: He quarreled with Resha'im due to the Pikudim, and not due to [their evil to] himself.
2.
Malbim: He quarrels with one who transgresses them.
2)
Why did he say "k'Chasdecha Chayeni"?
1.
Malbim: Aside from Mishpat that I requested, also do Chesed, for "Im Chasid Tischased" (18:26). To quarrel with them and risk my life for Kedushas Hashem is Chasidus, like it says in Brachos (20a) about Rav Yehudah.
