Is it wrong that I'm going to agree to share an apartment and then renege?
Why would you go back on it after agreeing?
I'm going to pretend I've found my own apartment
That is a mean thing for you to do. It's wrong to go back on an agreement you have agreed to. This person might not have been able to afford the apartment with some help.
I don't like how the atmosphere was getting tense now.
If you agree to share an apartment with someone, you should stick to your agreement. However, if you have a good reason for not wanting to share an apartment with someone, it's good to be honest as soon as possible.