Love can also be defined biblically as one who obeys the commandments of God. If that is a reasonable definition, then God showing justice to the wicked is actually love. Whereas if he just let them do whatever they want without regard to how they hurt, then that would not be love. Hence, I don't see a contradiction, on a number of levels.
How is it love to sacrifice your son for the sins of others?
That's a great question. But even in the modern world, the rights of the individual become subject to the rights of the many, it isn't that difficult to see. In China for instance, it is considered right and just and therefore loving to humanity as a whole, to subject the rights of the individual for the interests of the whole. Some might call this utilitarianism. the ends justify the means. It is loving for China to subject Taiwan to sanctions and pain. and slavery. For it will bring them back to the fold.
Of Course, the Bible's morality isn't based on utilitarianism, so I couldn't use that as an argument even if the world thought it was a good one.
The Concept of God sacrificing his Son is, however, very different to a human sacrificing his son on many different levels. It's not even close to a similar story in the Bible, where Abraham was commanded to sacrifice his son because God commanded him too. The Story of Abraham and Isaac is useful for many ideas but it is not the story of God and his Son being put to death.
I suppose the question you asked could be considered on a different levels.
You ask "how could it be love"? You didn't specify where the love is directed or not directed. If the question was - how is it love towards the people of the world who would be saved from certain death and hell by the sacrifice of this Son then the answer is not difficult. To save anyone from Hell or from certain death by any act that is going to cost you is a demonstration of love towards those people. Whether they accept it as love or not is irrelevant for - one of the definitions of love is sacrificial. Giving up your most valuable things for someone else's benefit.
If you were to ask whether the death of his son was love directed towards the devil, then I would say, it wasn't.
Of course, we also need to understand that God is Trinity. The Son is not a biological son or child like humans have. The Son is Truly God as the Father is Truly God as the Holy Spirit is Truly God. So when God sent his Son to die, we can't put it in the same manner as a human sacrifice. God also knew Jesus would rise from the dead. Jesus knew he would die but he also knew he would rise from the dead. And the purpose for his death and resurrection was for Jesus to gain a bride, the church, those who trusted in him. Therein is love by the Father for his Son, for he brought about his bride for him.
Jesus like many people in this world would sacrifice himself for the sake of others. That too is a demonstration of love. He didn't have to hold himself back. The fact that Jesus was prepared to die not just for his friends and family but also for his enemies demonstrates something more profound than love too. Doesn't it? IT demonstrates supreme integrity, grace and generosity, let alone kindness and mercy.
So how does it demonstrate love? Well, I would it by saying how does not demonstrate love? In fact to call into question Jesus' motivations is to call into question the motivations of every soldier who has ever fought and died for his country. And every police officer or fire person or first responder. It's a clear example. And to deny that is well to deny the reality of the world we live in.