If this is what you want to do, then go ahead. I really hope that you change your mind, though.
Since it is the weekend of the 15th, you still have some options! First, unless this guy is the department head *he won't even see that you signed your offer, provided you decline now*. Do you think every professor is involved in admissions? They will find out about admissions at some point in the future, but not immediately (and especially not on the 15th)
If you are uncomfortable doing this (although it seems to be, by far, your best option) then you can defer admission for a year and stay in the same city. Then re-apply next year and take a position in the lab you want to join. This is 100% legit and nobody is going to hold it against you. In fact, if this professor is the type to hold it against you, you certainly don't want to be working with him for the next 5+ years
Basically, it sounds like you are making a terrible decision. I really hope you reconsider