David sings over Abner and calls his death the death of a fool. Abner, who was a wise man had actually died like a fool.
2 Samuel 3: 33 And the king made a song of grief for Abner and said, Was the death of Abner to be like the death of a foolish man?
34 Your hands were free, your feet were not chained: like the downfall of a man before evil men, so was your fall. And the weeping of the people over him went on again.
Abner's death was called the death of a foolish man because he did not die in captivity. His hands were free and his feet were not in chains.
Abner should not have died in the way he did. He did not die in battle as would have been expected and which would have been considered more noble. No, he died by deceit. But he also died of his own lack of wisdom, his own foolishness. He agreed to meet Joab without considering Joab would still be carrying a grudge over the death of his brother Asahel. For a man who was so knowledgeable in the ways of war and diplomacy he died as one who had broken all of the rules he knew.
There is a spiritual application. Spiritual failure comes on many people in such a way to make them a fool. Sin overcomes us even though our hands are free and our feet are not in chains. Most people walk into sin and its consequences of their own accord.
We knowingly run the risks. We allow ourselves to open to the risks of sin. We conduct ourselves like foolish people indeed. We place ourselves in harm's way. Satan does not have to trap us because we set ourselves up for failure.
We let our guards down at the wrong time.