The first gate of Hell is hatred. As Paradise is the kingdom of love, so Hell is the kingdom of hatred. Father, says such a person, I am grateful to and love my friends, but I cannot endure him who does me an injury. Now, brother, you must know that the barbarians, the Turks and Indians say and do all this: Do not also the heathens this? says the Lord. To wish well to him who serves you is a natural thing; it is done not only by the infidel, but even by the brutes and wild beasts. But I say to you. Hear what I say to you says Jesus Christ; hear My law, which is a law of love: Love your enemies.
--St. Alphonsus Liguori. Six Discourses on Natural Calamities, Divine Threats, and the Four Gates of Hell