The poet asks his mistress to be cruel with wisdom rather than disdain, fearing that if driven mad by rejection, he might slander her—and fearing the world will believe him.
The poem pivots on the word 'mad' (line 9): if the poet despairs, he will go mad, and madness will make him 'speak ill' (slander) the Dark Lady. He's asking her to avoid driving him insane because he cannot trust himself once reason is gone. This reveals something dark: the poet is aware of his own volatility, his capacity for destructive vindication. He asks her to manage his emotions for him, to keep him sane by withholding the ultimate cruelty.
Lines 11-12 shift focus to the social world: 'Now this ill-wresting world is grown so bad, / Mad slanderers by mad ears believed be.' The problem isn't just the poet's potential madness but the world's readiness to believe slander. Mad people speaking to mad audiences will be believed. The poet fears that once he loses control, the world will accept his version of her cruelty. His plea, then, is simultaneously self-protective and a threat: drive me mad and I'll destroy you socially.
Like someone in a relationship texting their partner: 'If you keep ignoring me, I might say things I'll regret, and people will listen.' It's a threat disguised as warning, desperation dressed as plea. The poet fears madness more than pain.