David does not fear the sons of Zeruiah. The word used is קָשֶׁה ("qasheh") which implies difficulty or harshness; David is saying they are too fierce, and that he is neither complicit in nor condoning of their violent acts.
Recall that in 2 Samuel 3 David is dealing with a tense political development. Abner, who installed Ish-Bosheth as King of Israel (2 Sam 2:8-9), had been killed by Joab after David made a covenant with him for Israel's throne (2 Sam 3:12-30). The public reception of David, who seemed to have allowed his subordinate to kill a political opponent after assuring him of peace, is consequently at that moment placed in a fragile position.
David decides to demonstrate to the people in Hebron that he did not order or approve the death of Abner; David specifically uses the occasion of Abner's mourning to declare that not only has "a great prince fallen" but that the LORD will repay the evil done to Abner:
38 Then the king said to his servants, “Do you not realize that a great
prince has fallen today in Israel? 39 And I am weak this day [through fasting: see 2 Sam 3:35], though
anointed as king, and these men, the sons of Zeruiah, are too fierce
for me. May the LORD repay the evildoer according to his evil!”
(2 Sam 3:38-39, BSB)
The passage you quoted is rhetorical, meant to emphasize the responsibility of Joab and secure David's moral legitimacy. David upholds and honors his covenants.
Furthermore, all of the sons of Zeruiah are David's nephews:
Jesse was the father of Eliab his firstborn; the second son was
Abinadab, the third Shimea, the fourth Nethanel, the fifth Raddai, the
sixth Ozem and the seventh David. Their sisters were Zeruiah and
Abigail. Zeruiah’s three sons were Abishai, Joab and Asahel.
(1 Chron 2:13-16, NIV)
David does not immediately kill Joab for his transgressions not simply because he is a mighty man but because he is family.
However, at the end of his life, David instructs Solomon thusly:
5 “Now you yourself know what Joab son of Zeruiah did to me—what he
did to the two commanders of Israel’s armies, Abner son of Ner and
Amasa son of Jether. He killed them, shedding their blood in peacetime
as if in battle, and with that blood he stained the belt around his
waist and the sandals on his feet. 6 Deal with him according to your
wisdom, but do not let his gray head go down to the grave in peace.
(1 Kings 2:5-6, NIV)
Note that this is a completion of the foreshadowing in David's lamentation: Joab was repaid.
