Why is the inside of a cell negatively charged
Net Potential on a cell surface is always measured in comparison to the extracellular media...so it is a relative parameter. So inside of the cell membrane is negative implies that there is more negative charges inside or more positive changes outside compared to the respective opposite side of the membrane. As we do not deal with the "absolute potential" we need not calculate the values of them. 

K+ concentration is more inside the cell compared to outside...but there are other ions in and outside cells...e.g. Na+, Ca+,  proteins, Cl- etc...all of them together determine the potential difference across cell membrane.