The Buddha actually says it's not possible to have any ill will, aversion, or hostility, if there is mettā. Cultivating and developing mettā – getting the hang of it, understanding it and having it accessible – helps us be free of ill will. The idea is to make it accessible. Become familiar with it. Develop it as a habit. Practice it, develop it, and think about it, so that when you need it, it's available. You know how to find it in yourself, evoking it in a genuine way – a way that really has integrity, not something that's just a surface coating of ourselves.