Why? Because a belief system must be judged on whether it is true or false, not on the conduct (or misconduct) of the people who follow it. So as far as Christianity goes, one should look to Christ and the system itself rather than the followers, since they are imperfect people too.
Even if Einstein were a murderer, E=MC2 would still be true.
Even if Einstein were a murderer, E=MC2 would still be true.