Tell me, what is it you believe that hypothetical future dictator will do that is worse than killing billions?
The answer isn't limiting peoples' lifespans. Indeed, we've had autocratic rulers in our history who were considered excellent leaders, and people who lived under them experienced a "golden age". The problem was usually when they died, and their incompetent heir took over.
The answer is to come up with better governmental systems, not rely on limited lifespans. Doing the latter certainly didn't spare us from the horrors of the wars in the first half of the 20th century.
Here's the real truth of that: We don't let people get away with it for long. Point-like aggregations of power aren't easily workable. Even absolute totalitarians need help from factions and need to keep their minions well paid. Such rulers can't buy loyalty, and so depend on blood relatives for that as well, which means they are very constrained in this precious resource. It's only the wide dispersion of power that is workable and stable, long term.
https://www.youtube.com/watch?v=rStL7niR7gs
The wider the dispersion, the better off the common person and society.
People often further respond with something like: "I don't know about you, but I'd prefer that me and my loved ones die - the universe would get boring after so long". Of course, the vast majority of these people would still yearn for life even on their death beds at age 97 - and if technology and medicine allowed us to have sound minds and bodies by that age, the yearning would be even greater.
Who wants oblivion when eternal adventure could await us?
Consider that you will no longer need to procreate to have kids. They can and will likely be engineered. You'd probably not think the same way about families. Emotions could be something you can more readily control and so on...
I'm up for the challenge of trans-humanism, but I'm also okay with death if that's what comes to pass.
Certainly most people wish that they could relive their childhood, do things differently, treasure how carefree it is to be a child, without the responsibilities of adulthood. But that is not what childhood is: the ignorance of what is to come is what gives childhood its essential character. Life, likewise, derives its character from its immutability: our inability to experience more than one Everett branch. That immutability is what gives our decisions meaning: they are truly final.
Immortalism is, I think, an obstinate rejection of life's finitude. The immortalist would like to live ten trillion lifetimes, sampling all varieties of conscious experience. But this is not compatible with a single continuous lifespan. To properly experience all that reality has to offer, the immortal would have to "start over" countless times, erasing their memory and transferring their consciousness into every variety of body, human and non-human. What distinguishes this process from death as we know it?
In other words: maybe you already are the Immortal.
The answer I came up with: quite simply, the difference is that you don't ever die. Old memories fade away and are forgotten. But we have experience with that already in our limited lifetimes and a lot of it is not too horrible, compared to dying. If it's slow and gradual, and you always retain the recent memories that are relevant to your current life, this "continuous dying" may not be such a bad alternative.
If the richest man alive can pay whatever is necessary to preserve his own life, indefinitely, it stands to reason that some portion of his wealth and power would be permanently dedicated to preserving that privilege.
If the rich man withdraws from society, and dedicates some or all resources to preservation of the self, society then acquires an interest in supporting those who would murder him and take all his stuff, thus returning it to market circulation.
Prior to clinical immortality, you can always count on a change in estate financial management, because everyone who owns stuff will eventually die and cede control of it to others. You don't have to murder, because time and entropy do it for you.
We don't have immortality now, and actually we probably live longer that we ever did in the past. Yet our history is full of true horrors perpetuated by horrible leaders: just look at the horrors of WWII. These days, people are living longer, and we don't have those horrors any more (we still have some horrors, but overall things are much more peaceful than they were in the 1930s-40s). A cult of assassins could have really come in handy back then, to prevent 10s of millions of dead.
I'm sorry, I see no evidence to back this idea of yours that longer lifespans will lead to worse societies. If anything, the opposite seems to be the case. Back when people didn't live very long (e.g., Medieval times), wars were extremely common, and some of the larger causes of death were war, fighting, crime, etc. Now people in developed nations don't have to worry so much about these, and need to worry a lot more about Alzheimer's, Parkinson's, heart disease, and car crashes.
>If the rich man withdraws from society, and dedicates some or all resources to preservation of the self, society then acquires an interest in supporting those who would murder him and take all his stuff, thus returning it to market circulation.
This is why we have things like property taxes. You don't need to murder rich people, you just need to raise property taxes on people who have far more property than normal people. This can be done any time with laws. The only reason we don't pass such laws is because common people refuse to demand such laws, or to vote for people who would pass such laws. Blame the voters.
Rich people can and do seize political power for no greater purpose than to lower their own taxes, and possibly also loot the treasury in the process. The voters do want higher taxes on the rich. But the rich own the news media outlets, and buy the attention and gratitude of the legislators, judges, and prosecutors, and own the contractor businesses that provide elections services to counties and municipalities. Whenever someone asks, "why are we not raising taxes on the rich?", someone has already been well paid to divert the conversation into an acrimonious distraction. Note that the wedge issues commonly used for this purpose are universally of low importance to the wealthy class.
What makes you think that the rich will pay what the poor ask of them, when they have no effective means of enforcing their will?