What will Tzadikim see?
Radak: They will see Hashem's vengeance for the Kohanim's blood, and fear Hashem more.
Malbim: There was a punishment via Hashgachah Midah k'Neged Midah. Do'eg was Sha'ul's Shali'ach to kill Kohanei Hashem. Now (refer to 52:6:2:2), he said about himself that he was Sha'ul's Shali'ach to kill Mashi'ach Hashem (Sha'ul), for "Achazani ha'Shevetz" (Shmuel II, 1:9), i.e. the sin of killing Nov Ir ha'Kohanim. Shevetz refers to Bigdei Kehunah 1 - "v'Asisa Mishbetzos Zahav" (Shemos 28:13). David killed him; due to David, Do'eg killed the Kohanim!
Tanchuma (Metzora 4) explains so. The simple meaning is that Sha'ul was seized with trembling before death. (PF)
What mockery will they make of him?
Rashi: They will say, he did not put his trust in Hashem (verse 9). Look what happened to him!
