Wow, how compassionate of you.
If a country had 100m population 20 years ago then there will be say, 15m people now in their last decade with ~1.5m dying per year = 1.5% of the original population. But if the birth rate has been high for those 20 years then the population might now be 200m so those 1.5m that die this year only constitutes 0.75% of the current population.
And conversely, if the birth rate had been very low for those 20 years then the current population may have dropped so those 15m people to die this year would represent more than 1.5% of the current population
Plus, I'm not sure how they are counting the populations as many Easter Europeans, often the younger generations, have moved west in the last 30 years so populations in that part of the world have aged faster than elsewhere
If the country has all four of these, they're at the top of the list.
If they don't have the first two things, but they've got a lot of the last two things, they're mostly near the middle.
If they have lots of the first two things, but don't have the last two things, then they're mostly at or near the bottom of the list.
It's more to do with birth rates of more recent generations, migration, etc
Also birth rate is extremely low, so not much population replenishment either, which means by each year that passes, the higher percent of these countries populations will die.
African countries like Chad and Sierra Leone is mostly due to lack of good healthcare. In Chad, almost all doctors are imported from France, cause Chad doesn't have the education system to educate doctors or other healthcare personnel.