Look, I know Mrs. White a fictional character, but you really should look up tact in the dictionary and keep it somewhere in your mind palace. You know, to avoid offending someone else along the line.

[ like he did with molly hooper. and him. and that poor woman on the tube who was expecting twins. ]

I still think the library is the best place. It'll be quiet because this is a party and unless they're… you… no one is going to be in there. They'll all be playing snooker, talking in the lounge or dancing in the ballroom. Actually, the kitchen would make a good place too. If there's a big freezer, the murderer can leave Mr. Black in there to make the time of death unclear.