Hi Annie and Welcome to the forum.....
I have grown Pencil
cactus(plants) for a long time,,,,.
It could be there wasn't enough air circulation in the corner,, as why they backside isn't happy right now,,,,
I've never had that happen so I'm trying to picture how it may look,,,,,, I don't know what area you live in either, so I can't say to take outside,,,, But I wouldn't water it too much,, ( I don't know how wet is was before you watered it today
)
You didn't say if any of it was very soft ,, l But.,.
Any branch that is almost "Mushy" should be cut off,, be careful the milky liquid can me toxic,, wear gloves while cutting,,. Just in case, :wink:
Make sure there is enough air ciculation and sunny place for it to heal,,
I am not sure if you have actually killed it although you have not been kind, I guess.
I wouldn't say you've not been Kind,,,,,
In fact you
were being kind to it in considering a safe place to put it while company was there,
One thing to remember,, Air circulation is important to all plants .
Can you take A picture of it so I can see,, then maybe I could assess the problem better>???/