Then, it's possible that nets are being distributed by evil people who make their victims kneel for hours before getting help. (This is extreme, but it could involve things diametrically opposed to your values; maybe Islam is being spread in traditional animistic societies[0], destroying traditional culture; maybe they're micro-chipping these people.) Cost-effectiveness is a good metric, but if you know nothing about what's involved in curing malaria...
Exactly the reason everyone should do research before giving to charity. Since unlike products you buy, which you can test out and even return, charitable donations provide you with no feedback, you must research charities.
The great news is GiveWell has been doing this for over a decade (full time!) and has excellent recommendations.
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[0] Semi-relatedly, https://en.wikipedia.org/wiki/The_Save_the_Children_Fund_Fil...