Sparked by this: http://www.albertmohler.com/2013/07/30/who-am-i-to-judge-the-pope-the-press-and-the-predicament/
Given: All persons are to be respected as persons.
Given: There are actions which can be said to be bad, at least beyond a shadow of a doubt (e.g., killing another without just cause).
What is bad is to be regarded as something to be done away with. Persons, as persons, are not to be done away with. To regard the sin as to be done away with is good, but to regard the sinner as to be done away with is bad. Thus: love the sinner, but hate the sin.
This is a relatively simple proof of sorts for the principle, but for a proper defense I would like to show why the most obvious alternative is unworkable.
To regard persons as the sum of their actions and desires is to diminish them as persons. It is to say "you are nothing more than the one who did these things and wanted those things." Yet we want to add that persons are not only those who do and want, but also those who need. At our best, our desires match our needs. Very often, though, we want what is not good for us. To love the sinner we must hate the sin, because sin is bad. Sin is bad not only in itself, but for the sinner. If we ignored what people need, we could easily argue that to love a person we must endorse their choices as good. That was Sartre's philosophy--that what a person does is called good by them, and that, therefore, a person can always say "What I want is good." It is not, however, a livable philosophy. We all are aware of things we would change about ourselves. Thus, to follow Sartre's philosophy we must argue a sort of contradiction: I want to change my actions (I feel that other actions than those I do are good) and what I do is what I should do.