It's the cruelty that's difficult to bare. Life comes with a spectrum of experiences, elation, joy, misery, hatred, in existence we encounter a bevy of emotional states. This is the nature of human life, and to experience only one side of it robs you of it's value.
But cruelty is the point where we must draw a line, where we have to adjust our behavior when we feel it begin to arise within.
When we desire the suffering of others, we bring upon a suffering amongst ourselves. Cruelty is the intent behind that desire to inflict suffering. It's what gives rise to hatred, to violence. It exists long before either can appear, and without it, neither can occur.