Question *Spoiler*

Good question actually. But I think it was perhaps so that he and Jane Appleby could maintain their ruse and their cover so that they could get rid of Penny along with her father's body by pushing the car over the cliff later on. But yes, it seems that it would have been easier to let Penny drown in the pool and then get rid of the father's body later. Either that or it would have been too hard to explain to the authorities why no one saved Penny when they were quite clearly present at the home at the time of her falling in the pool. That's my 2 cents anyway. But again, I think you raise a good question. But isn't this a great movie though! One of my favorites ever since I first saw it as a teen on the late late late movie while growing up in Melbourne Australia.

Watch Bedbug on YouTube at
http://www.youtube.com/watch?v=5QI_1YSXt8Y/

I just remember that the initial plan of them was not to kill her and just make her crazy, because they first try to scare her with the corpse of her dead father.

Yes they could have let her drown then. But guess what, she wouldn't have drown because she wasn't really crippled. So what would be the point of the movie showing that happen. Then they (Bob & Jane) would start to be suspicious.

The stepmother didn't want anyone to DIE, if she could help it. She didn't even know that Robert had killed her husband until much later on in the film - she had been led to believe that it was an actual accident by Robert. I'm sure Robert would have been only too glad to let Penny die in the pool, but Jane wouldn't have allowed it at THAT point in time.