What is love? Love is a sufficiently strong and long lasting attitude that effectively brings a great impact on behavior.
Casual liking and reactions are limited in level and don't last very long in comparison to love.
It is absolutely true that, some people are incapable to love. They are capable of actions that cause others to matter to them; they can be skillful enough to be devoted to someone; but their attitude is not strong enough and don't last long enough for us to call it love. They may pretend to love someone, but their actions for this person expose their true identity.
They cannot love, so they are lacking in emotions. The very few emotions they have are often very superficial. They are indifferent in interpersonal relations. They are incapable of putting themselves in someone else's shoes and understanding what is going on for the other person. No matter how well they are treated there is never any stable response of gratitude.
They don't feel any obligation to respond normally to kindness or love. They don't find anything emotionally meaningful in the relationships in comparison to average people. This makes them unresponsive to the pain and suffering of others. In fact, they often feel superior because they think that they are not loaded with or limited by emotions.
A strong sense of power, a massive ego, thinking they are superior and better than others, makes them very selfish and often means they will do whatever they like, sometimes just because they can.
They have no sense of right and wrong, so without feeling the smallest amount of pity/regret or guilt they will make decisions to mentally/emotionally mistreat others, no matter how much suffering they have caused to the other person.
Note: This is a work of study, do not take it personally. :)