The resistance fighters said in a statement that they had kidnapped 51-year-old Sasson Nuriel, who the Israelis say was a businessman, on Wednesday.
They had intended to trade him for Palestinian prisoners held in Israeli jails.
However, after Israel began a series of arrest raids against West Bank resistance fighters, they decided to kill him, the statement said on Tuesday.
The group said it had identified Nuriel as an agent of Israel's Shin Bet security service.
Nuriel was found dead on Monday near the West Bank town of Ram Allah, and police said the attack appeared to be a "terror attack".
Public Security Minister Gideon Ezra said on Monday that security forces had in recent days arrested several Hamas members near Ram Allah on suspicion the group had kidnapped the businessman.
Police said a Hamas activist arrested on Sunday led the forces to Nuriel's body in an industrial zone in the West Bank town of Beitunia near Ram Allah.