A cat broke into my house once too.  It stayed in my room for 2 days before I noticed it.  (Tells you how clean my room is, and how well used most of it is).  I heard growling now and then, but I assumed it was outside my window, and didn't really do anything about it.  One day I was going to bed though, and I heard a growl coming from underneath my bed.  Now this I could hear perfectly well.  I looked under, and there was this huge grey tomcat underneath my bed.
I tried to catch it, but it ran away through a hole in the wall (tells you how good of condition my room was in).  The next day I found it there again, and I was determined to get rid of it once and for all.  I snuck up on it ninja-style, and grabbed it by the back of the neck.  It turned around and scratched the sh!t out of my arm, but I didn't let go.  I instead let go of my grip on the back of its neck and instead starting choking the little prick.  As the cat struggled for air, I threw it into a nearby box, and sealed it tight.  I was intending to take it out back and finish the job, but as I was taking the box out the back door, it got out.  I threw a few rocks at it, but it was gone before I could hit it.
That was one horrible little bastard of a cat.