Why not? He can sink a warship and the only answer is pay the man. He can shell an island and the only answer is pay the man. I can accept that you can conceive of a line, somewhere, that North Korea could cross and you'd theoretically agree that subsidizing him is not a good idea, but subsidizing him allows North Korea to move that line back. Maybe it's not using ten nukes and holding twenty, maybe he can only use one or two and hold thirty; if he nukes, say, Hyangsan but immediately declares he has thirty or forty more nukes aimed at Seoul and Tokyo and every major city in South Korea and Japan, are you really going to flip and say okay, now he's gone to far? If so, is it just nuclear weapons? Is there also some magical number of people he can kill in a given period that would make you withdraw your support?
Either way, subsidizing him certainly allows him to become more powerful, more quickly.
Nuclear weapons are certainly different, and they have been for a very long time. If North Korea nuked South Korea or anyone else (except for maybe Iran, haha), we would obliterate them either through conventional or nuclear means. What stops people from nuking their opponents now is basically a gentleman's agreement that nukes are off the table until someone uses them. Once they do, all bets are off.
I have no idea the number of people it would take for me to decide that foreign aid was not worth it. It really has far less to do with the number of people and more to do with the circumstances.