Whom did Yehu kill?
Radak: He killed all Ovdei ha'Ba'al and Beis Achav.
Where do we find that Elisha killed people?
Rashi: He caused bears to kill 42 youths in Yericho (Melachim II, 2:24).
Radak #1: He rebuked Yisrael, and they did not heed him; enemies came and killed them. It is as if he killed them. Had they not been warned, they would not be punished so harshly.
Radak #2: He killed via his death 1 ; when he died, troops of Aram and Mo'av came (Melachim II, 13:20).
Why is this considered that Elisha killed them? It seems that his merit protected them, so they died only after he died, and they lost his merit! (PF)
Hashem commanded him to anoint Chaza'el, Yehu and Elisha (verses 15-16), implying in this order. Also here it implies that first Chaza'el will kill, and then Yehu, and then Elisha. They were appointed, and they killed, in the reverse order!
Radak (15): Eliyahu will make Elisha Navi in place of himself, and command him to anoint them 1 .
Malbim: Because Achav repented, the order was changed. Yehu will not kill the sinners in Achav's lifetime, rather, in Achav's son's lifetime. Also the decree of Chaza'el was detained. "Those who survive Chaza'el's sword, Yehu will kill them" (verse 17) was the initial decree; afterwards, the order was reversed.
Malbim (15, citing Mahari): Those not proper to fall (die) via Chaza'el, e.g. Beis Achav, women and children, will fall via Yehu. Those not proper to fall via Yehu, e.g. the youths who insulted Elisha, Elisha will kill them (Melachim II, 2:24). The verse does not discuss the order in which the three will kill people. 2
Radak (15): If so, why did Hashem command him to go to Damesek? He was going, and on the way he saw Elisha, who was the last one he was commanded to anoint. He understood that the others will be anointed via Elisha. Hashem mentioned them first only because they will be first to avenge (kill the sinners on Yisrael).
The verse says "ha'Nimlat" (who escapes)! Perhaps this refers to Din. Hashem judged that they do not deserve to die via Chaza'el (or Yehu), so they 'escaped' this; Hashem rather decreed that they die via Yehu (or Elisha). However, why is dying via Elisha better than via Yehu? Refer to 19:17:2:1. (PF)