I actually felt sorry for the guy who couldn't swim.I mean he panicking and instead of helping him Becky kills him with the boat.If she had helped him he would be grateful and might have helped her.
He was associated with the group that killed her dad and one of their dogs. Her experience was that they were murderers and liars. It feels sad because the guy was helpless, I will agree to that. But the danger he posed if he ever got out of the water was too great. That and she wanted revenge for what they did. It sounds messed up, but I think it was the best choice she could have made considering what she knew at the time.