I personally find it hard to admire someone whom I know was explicitly anti-Catholic, but there are a number of people whose good qualities I admired long before I knew this about them.
My signature is and has always been a C.S. Lewis quote. He was Anglican and resisted conversion to Catholicism until the day he died. Does this make me admire him any less? Not one smidgen. Of course, I don’t think anyone would call him “anti-Catholic,” but you see my point. He will always be one of my heroes.
All who have achieved something great have qualities that we can admire. Be it perseverence, or great faith, or focus, or what have you. These are all God-given gifts, even if they remain uncompleted by the fullness of the Truth. Even the major world religions have a piece of the Truth to offer, which ought to be valued simply for being Truth.
Praise God for the good He has shown through His creation, and pray for the corrupted parts. Then there is balance.