I'm back reading here, but wanted to answer this.
The insurance policy paid out if she were paralyzed from an accident, so the original plan was for Stan to shoot her in the spine.
He thought that was too dangerous, so instead he punched her in the face and hit her ankle with a sledgehammer.
It's odd since those injuries wouldn't lead to paralysis so there really was nothing to gain from doing that.
I honestly do not understand that thought process, but when two crazies get together amazing things can happen.