King David was a legendary Warrior himself (apart from slaying Goliath, he slayed many others):

“As they danced, they sang: “Saul has slain his thousands, and David his tens of thousands.” (1 Samuel 18:7)

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He has even killed a bear and a lion:

“But David said to Saul, “Your servant was tending his father’s sheep. When a lion or a bear came and took a lamb from the flock, I went out after him and attacked him, and rescued it from his mouth; and when he rose up against me, I seized him by his beard and struck him and killed him. “Your servant has killed both the lion and the bear; and this uncircumcised Philistine will be like one of them, since he has taunted the armies of the living God.” (1 Samuel 17:34 – 36)

One of his most emotionally charged revenge was due to his enemies slaying ‘his best friend’ and the first prince of Israel, Jonathan whose love, David implies, “surpassed the love of women”:

“How have the mighty fallen in the midst of the battle! Jonathan is slain on your high places.

“I am distressed for you, my brother Jonathan; You have been very pleasant to me. Your love to me was more wonderful than the love of women.” (2 Samuel 1:25 – 26)

P/S:

And after his period of grieving, king David avenged a ‘clean sweep’ of those enemies who slayed ‘his brother Jonathan’ (David also killed the internal enemies in Israel who were of the house of Saul* prior to this period).

“So David did as the Lord commanded him, and he struck down the Philistines all the way from Gibeon to Gezer.” (2 Samuel 5:25)

*King Saul (the first** king of Israel) was Jonathan’s father. Though Jonathan was the first prince of Israel and the rightful heir to the throne, at the command of the LORD, not only he did “not” oppose David to be the next King of Israel but also “helped” David in many ways whilst being a “brother” to him.

**Prior to this, Israel didn’t have kings for God was their King. However, the people demanded for an earthly king as well (as it was with the surrounding nations) and God granted their wish.