The speaker admits to a surprising weakness: a heart that can't be cruel. The title and opening line join up, so the poem reads "I don't have a heart for cruelty." The speaker doesn't think cruelty is always wrong. In fact, they say some situations call for a harsh, decisive blow, one that leaves the other person weeping and pleading without a single kind word. Doing that, they suggest, takes a cold, reptilian heart of stone.
The speaker isn't built that way. They always give a second chance, and they call this their "fatal flaw," something they expect will bring about their end. The closing image makes the point sharply: they would rather forgive a snake for biting than cut off its head. The mood is rueful and resigned. There's some self-criticism here, but also a quiet pride in being gentle, even when it may cost them.