Don Murphy reminds me that John Wilkes Booth wasn't put on trial for shooting Abe Lincoln. Booth was tracked down and killed by Union soldiers twelve days after his foul deed.
So we're left with Spade Cooley and Phil Spector as the only two known celebrities to be convicted of murder. I suppose one could argue that neither was that much of a celebrity at the moment of their respective killings. Then again, neither was O.J.