Sometimes the most loving thing you could possibly do is to call someone a slut, for example. That is, as long as it is the truth. Calling someone a slut or a whore just for the sake of getting some frustration off your chest is a hurtful and wrong thing to do, but if some girl is sleeping around and going to clubs to pick up guys or to be picked up by guys, then she is a slut and needs to be told so. It only harms her further to be nice and try to be her friend and do "lifestyle evangelism". That sort of crap doesn't work and it's not Biblical.
The law is a tutor to bring people to Christ. It is the conviction that the law brings in the heart of the sinner that communicates to them their need for forgiveness; their need for a savior. The most loving thing to do for sinners is to communicate judgment and guilt to them via the law. That's what the law if for, that's what Jesus did and that's what we should do also. It isn't nice, but it is kind and loving. Most people would say that it is hate, and in important ways it is just that, but it is also love because you do hate them in hopes that they will repent because your hatred told them that there was something wrong with them and they will therefore not spend eternity in Hell.
They do not, by the way, usually repent while the person who hates them is sticking around. That harvest is for someone else to reap other than the one who sowed the seed of guilt and conviction in the heart of the sinner. There are exceptions to this but not many at all.