Edward Ates says he didn't have the energy to accurately shoot Paul Duncsak and make a quick getaway. The 62-year-old was 285 pounds when Duncsak was killed in 2006.
The victim and Ates' daughter were involved in a bitter custody dispute after their divorce.
Ates testified that he had no reason to want Duncsak dead.
Prosecutors say Ates drove from Florida to Duncsak's home in Ramsey, New Jersey, climbed a staircase and shot the 40-year-old pharmaceutical executive before feeling to Louisiana.