It’s often assumed that mindfulness has kindness built-in. I don’t think this is true. A counter-example is the way that some beginner meditators criticise themselves for not being mindful enough. It’s an act of unkindness towards one’s self.
Mindfulness needs to be deliberately filled with the quality of goodwill.
As well as making a healthy static meditation practice, kindness can also be used to give a sense of direction to the practice. It’s the first signpost that we are moving along in the direction of less suffering.
Mindfulness without kindness is like standing on one leg. It won’t get us very far along the path.