Ishbosheth was David's rival. Why did David have the men who killed him put to death?

David gives his reasons in his own words, and the chapter adds none of its own. He begins, "As the LORD lives, who has redeemed my soul out of all adversity" (4:9). He reminds them of the young man who came to him at Ziklag: "when someone told me, ‘Behold, Saul is dead,’ thinking that he brought good news, I seized him and killed him in Ziklag, which was the reward I gave him for his news" (4:10). That is David's own account of that day; chapter 1 doesn't say whether the young man's story was true (1:10, 1:16). Then David says, "How much more, when wicked men have slain a righteous person in his own house on his bed, should I not now require his blood from your hand, and rid the earth of you?" (4:11). In chapter 1 his words were about "the LORD’s anointed" (1:14, 1:16). Here they are about "a righteous person" killed "in his own house on his bed" (4:11); the chapter doesn't call Ishbosheth the LORD's anointed. Beyond David's words, it tells only what was done (4:12), and this answer doesn't go past them.