You might respect an older person in shul, but you dont necessarily love them and on the flip side, as has been mentioned already the mother of a serial killer still loves her child although she doesnt respect him.
I think the basic point made here, is that even if you vehemently disagree with someone, the way to make your point is respectfully. When you say a polite good morning to your not yet frum neighbor, do you love him, or do you simply treat him like another human being. Nobody is suggesting giving him shlishi, but polite good morning is in order, he is still a tzelem elokim, albeit on the wrong track. How many people nowadays would actually fall into the “mumar lihachas” category?