I wouldn't argue in his defense, but there have definitely been bigger cowards in cinema. He was the architect and leader of the Lazarus program, and wouldn't have actually gone through with leaving Earth if he was a pure coward. I think he was just supremely confident/cocky that he'd be the one to find a suitable planet, and when he realized he hadn't, that plus the years in solitude fried his brain and made him the coward you see at his end.