Mettā needs upekkhā or equanimity if it’s not going to turn into a source of suffering.
"Equanimity is the most refined of all the brahma-vihāras. It’s not a wish; it’s a statement of fact. All beings are the owners of their actions, heir to their actions. Whatever they do, for good or for evil, to that will they fall heir. Period. It’s a chastening thought, but one that you can hold on to and not suffer nearly so much. Ajaan Fuang commented that mettā needs upekkhā or equanimity if it’s not going to turn into a source of suffering. This is why the brahma-vihāras come in a set. Mettā keeps upekkhā from becoming cold and heartless. You go through the first three before you get to the fourth one. The equanimity is where you can go when you look at the world and realize, “They’re hunting people down and they’re making it legal. People in power are ripping off the rest of the society, and it’s legal.” And you want to wonder, what’s wrong with this human race? So you have to step back a little bit further and say, “Well, it’s karma.” And where are you goin...