Those who accept eternal life look for happiness and perfection in God; in the Kingdom of God. Those who think there is no eternal existence can only look to this mortal world for happiness, so they want to try to make this world as “perfect” as possible (noteworthy that various ideas of what constitutes “happiness” often are contradictory).
The attempt to make this life “heaven on earth” has caused and will cause more misery than any single other idea or philosophy. A subset of this is the fact that there is alwasy one or more segments of people who will not want to go along with the “let’s make earth better and maybe even perfect” so of course those who don’t want to go along with the plan have to be subjegated, re-educated or killed. Just to make everyone happy, of course.
You really nailed it.
And we can see the reaction of those who want to make the world a better place and get frustrated in that attempt. They end up not making the earth the place they claim they want to.
The same can be said about various ideas of what constitutes "heaven" or "paradise."