If you were obviously a murderer, and then publicly railed against murder, it would make it seem suspiciously like you were up to something, which would tend to

If you were obviously a murderer, and then publicly railed against murder, it would make it seem suspiciously like you were up to something, which would tend to make people doubt what you're saying. It doesn't make what they're saying automatically invalid, but knowledge that the person saying something is obviously a hypocrite is a form of evidence about how likely what they're saying is true.