I know it is very mainstream among prissy left-wing Westerners to decide that those who disagree with them should die
You seem to have misread what he was saying, because he didn't say that.
Had most of Africa avoided New Imperialism, what would the continent look like?
Countries/Nation-States, Ideology, Demographics, Technology?
This would have huge implications for what the whole world looks like. Europe especially. A large amount of blood and treasure was sunk into conquering parts of Africa by European states, and the political repercussions of European imperialism on the continent had important effects on the politics of Europe.
That said, let's imagine that magically things do go mostly the same in the rest of the world, there's no way they would of course, but if we try and figure out how the whole world is changed we'll go way into the weeds here.
So, what would Africa look like? Well, things won't stand still without direct imperialism extending over almost the whole continent. For a start, the slave kingdoms (who had grown rich by depopulating their neighbours and selling them to European and Arabic traders) are still likely to go through a period of turbulence as the British do their best to shut down the transatlantic and Indian Ocean slave trade. Some will fall, some will survive. The new kingdoms that rise to replace them and the renewed survivors will be as plugged into trade with the European and Middle Eastern worlds as they were before, so likely we will still see the rise of palm oil plantations in what would in OTL be Nigeria, other cash crops like rubber, cocao, coffee, cotton etc also are still likely to rise. Only in this TL, it will be local elites forcing people into serfdom or outright slavery to grow more cash crops, and it will be local armies buying European guns to conquer their neighbours and force them to grow more cash crops. There will still be energetic European missionary activity, there will still be increasing technological flows and economic ties.
So comparing the situation to OTL South America isn't too far off I think. Local elites exploiting peasants who mostly grow cash crops and occasionally mine things to feed into an increasingly globalized world economy. I have a hard time imagining any of these African kingdoms avoiding heavy European influence. This won't be a utopia and this won't completely avoid the positive and negative changes that happened to Africa in OTL.
On balance I'd say that by 1950, Africa would be dominated by a patchwork of large and small African kingdoms and a few European colonies from the first wave of colonialism (Cape Colony, Algeria, Mozambique and Angola - which in TTL are likely even larger than OTL, the various French, British and Spanish trade outposts). Some of the African empires may actually face decolonization movements of their own over the course of the alt-Cold War period (the idea that there'd even BE a Soviet Union, let alone a Cold War is of course absurd, but we're just going with it for the sake of argument here). I suspect that the economy and level of infrastructure development in this alt-Africa would be higher - the rulers of the African kingdoms are likely to be an unpleasant a lot as you got with any gunpowder warlord states (see the history of Persia under the Safavids and India under the Mughals and of course the Ashanti kingdom from the earlier phase of African history), but unlike the British or the French colonialists, they need to live much closer to the people they're exploiting. So likely there would be some glittering capitals which could form the nuclei of industrial cities, a few more roads and railways built, more Africans (likely members of whatever tribal group was dominant in a given cash-crop serf kingdom) have world-class educations since it is hard to see the local kingdoms paying many French and British bureaucrats to come in and do all the paperwork (though there would be SOME of that going on).
I don't think that the spread of vaccination and Christianity would be significantly slowed compared to OTL, and either may even be faster.
Oh, and probably there's more slavery within Africa, or at least there is more open slavery (as opposed to situations where people are treated as slaves but were called by some euphemism). In OTL, a major driver of the expansion of the British empire in Africa was the effort to end slavery. So if there is no second wave of colonial expansion in Africa, that seems to imply that the British have decided that what the Africans do to each-other isn't their business (except where they can profit off of it in the form of cheap slave-grown rubber, for example).
And in Europe, I think things would be a little better also. European investment and unequal "alliances" will mean that Europe will get the benefits it got in OTL, but without the diverted talent, wasted money, deaths to tropical diseases and African resistance of OTL. Chances to avoid something like WW1 are better. The continent is probably at least a bit more developed and wealthy than it is in OTL and possibly (if there is nothing like WW1 or WW2) alot better off.
fasquardon