John Gill Commentary 2 Samuel 4:10

"when one told me, saying, Behold, Saul is dead, thinking to have brought good tidings, I took hold of him, and slew him in Ziklag, which was the reward I gave him for his tidings." — 2 Samuel 4:10 (ASV)

When one told me, saying, behold, Saul is dead
No more is related, not that he killed him, or assisted in killing him, only that he was dead; by which it appears, as Abarbinel thinks, that the Amalekite did not slay Saul, and that David did not put him to death on that account, but for what follows:

thinking to have brought good tidings ;
which would have been very acceptable to David, that he would have rejoiced and exulted at it as he did; but he was mistaken; instead of that,

I took hold of him, and slew him at Ziklag ;
that is, ordered one of his young men to lay hold on him, and slay him, as he did, (2 Samuel 1:15) ;

who [thought] that I would have given him a reward for his tidings ;
a handsome present, as the Targum here, a gift, or raised him to some post of honour and profit.