Revealing mistake: When Alex is walking back from Mrs Hess' after shovelling her drive, you can see the "crooks'" car lights waiting for their Q to enter the street in the top left. (00:10:10)
Revealing mistake: It's obvious the snow is fake. You can see when Alex runs outside in his pajamas.
Answer: It was Jernigan who thought there was a woman in the shower and he couldn't take the risk that if it was a woman, that she would eventually get out and see intruders in the house and call for the police. The only option open to Jernigan would be to kill her so there'd be no witnesses.
No it was Beaupre who thought there was a woman in the house.
I've seen the movie and I can confirm that it was Jernigan who opened the shower curtain because he thought a woman was in the shower.