Because of:
"No one knows exactly what happened to Lucy on the night she died, except very few people - even the person responsible for her death."
and
"We want people to say, 'Oh, that makes sense, that's upsetting and it's very clever'."
I think it was a Beale who doesn't even remember.
I did think maybe Lee could have done it in a PTSD episode but I doubt the armed forces would like that as it suggests that people suffering from PTSD are dangerous and it may prohibit their recovery.
So:
1) Peter
2) Ian
3) Lauren (outlier, not sure why as she hasn't been drinking, but would also tick the "upsetting" box)