The end (solved food shortage) does not justify the means (mass murder).
The means must be able to stand on their own.
No, there's a distribution problem, but I'll play along.
> One way to solve this would be to kill 80% of the African population.
Nope. Killing 80% of the African population will not solve the food shortage. (It also won't solve the distribution problem.) Why? Because Africa's food production is roughly proportional to its population and not limited by anything that will be helped by killing lots of people.
The "end" of killing 80% of the African population is that 80% of the population has been killed. Some "good person" might have thought that some good (less starvation) would result from killing them, but the end was a lot of dead people who got killed by said good person.
Thanks for providing an example of where "he meant well" had a horrendous result, as it typically does.
If the ends don't justify the means, what does?
The problem is actually on the other side. The means produce the end. Not intentions, not the morals of the persons involved, but the means. Fascism, for example, is the result of fascist acts, no matter who does them or their reasons for doing so.
That fact seems to be lost on "good people".