Good line of thinking. The killer is from within. Otherwise why should a killer bother to drag the body to terrace, cover it with cooler cover and lock the terrace again. Whoever may be that killer, he or she has taken enough time without hurrying. If there was any other killer they would have bolted Dr and Mrs. Talwars' room. A person who knew where the keys to terrace was, should also be knowing their bedroom.
