In Leviticus 19:17, God warns us against hating others: “Do not hate your brother in your heart.” Interestingly, that verse shows us that hate isn’t just an action; it is also an attitude of the heart. Many times we hate someone in our heart, yet pretend to like them. However, hating someone in your heart is nonetheless hate — and thus despised by God.
Although it sounds harsh, 1 John 3:15 says that anyone who hates his brother is a murderer. Indeed, as far as your heart is concerned, to hate is to murder. In other words, hating someone is no different than murdering them in your hearts.
If you hate someone, you’ll be thinking negatively about them, slandering them (if not verbally, at least in your heart), and cutting them down with your thoughts and words. Thus, it is no surprise that God sternly warns us not to hate others, because life is too short to spend it hating.