Kindness puts people at ease, tells them you're out to do them good, instead of harm. That allows people to feel safe around us. But if Ephesians 4:32 is right, it's mostly a matter of doing to others, as God has done for us.
Ancient gods are not usually portrayed as being kind. Their stories are more about anger or about being distant or uncaring, or doing weird things just for sport. That is how most of the ancients thought of their god(s). The Hebrew (and thus the Christian) God is characterized by love and kindness. But God is not always kind, due to God's sense of reality and justice. God will not turn kindness into a lie, and will only be kind in the face of deception and brutality for a short while.
Since God is kind, the follower of Christ is to be kind. Being kind is part of being holy, a mark of what makes the Spirit-led person distinct from others. How does this show?
•treating people with respect instead of cussing them out,
•listening instead of blowing them off,
•some gentle words for your girlfriend,
•a helping hand to an elderly woman in a store.
Do acts of kindness for people. Give them little tastes of God's love. People actually like being treated kindly, as if they are valued. They can lay down at least some of their defenses, relax more, and feel comfortable. Most non-Christians have an expectation of being treated a bit more kindly by Christians, and are disappointed, and sometimes even angry, when they're not treated that way. While it's wrong to live according to others' expectations, it is a challenge we need to take to heart. Many practicing Orthodox Jews today, do good deeds (mitzvot) as the opportunities present themselves in their daily lives, mainly because it is what God wants them to do. To them, doing good for others is more than a duty, it's also a prayer and a devotion.
Being vicious can be fun. There's a visceral kick one gets from throwing knives at one's chosen enemies. Radio talk show hosts, including some supposedly "Christian" ones, live on it, and their fans feed off of it. Gossip circles spread the ugly words far and wide. Put-downs become an every-day part of life. But cruelty doesn't make you important, it just makes you cruel.