Oh Sprinkles, you're so naive. You don't just kill the unhappy and the causers of unhappiness; you kill everyone. No people, no unhappiness.
But I suppose Lark was looking for an idea that doesn't result in the extinction of the human race. So my other suggestion would be mandatory lobotomies.
I see your logic, no, I'd not suggest for a moment that lobotomised individuals are happy in the terms under discussion, you could as easily say that with the spread of dementia we're all destined for happy end states anyway.
Neither is there any suggestion that happiness be the sole or only criterion for human life, elsewhere I think I wrote about experiencing sadness and expecting sadness and other difficult or challenging emotions as part of being, indeed that they can, properly understood as a state which is not permanent but only temporary and from which there can be relief, even if it is eventual rather than easy or immediate, be enriching of life.
However, considering, and I put this in the OP, that happiness is accepted as a worthwhile pursuit at all, then what makes for it and a happier world? Do you see what I'm meaning? There's of course questions to be asked about the first principles or assumptions in any discussion but without a certain acceptable axiomatic reasoning you cant have any discussion in the first place.
Bertrand Russell learned that when considering mathematics, his life's work to over turn axioms in the pursuit of rigour only resulted, eventually, and he acknowledged it as such, in some more valid axioms.