[cfgavplv] Everyone kills everyone

Hypothesize that every technological civilization gains the ability for 1% of the population to kill off a random 1% of the rest of the population.  At that point, the entire population will quickly be killed off, because it only takes a logarithmic number of people exercising that capability for the expected number of survivors to be zero: p*0.99^n tends to zero very quickly.  The number of people able to mount such an attack, 1% of the population, grows exponentially (assuming exponential growth of the total population), so efforts to prevent such attacks become exponentially difficult.  For our current population of 7 billion, 2256 successful attacks among the hypothetically 70,000,000 capable people (so despite thwarting 99.997% of attempts) would suffice to kill us all.

Few current weapons have the ability to kill off a given proportion of the population instead of just a constant number.  A bullet or an atomic bomb each kill a only constant number so aren't deadly enough.  Furthermore, the number of people able to unleash a nuclear attack, if we assume to be world leaders, remains constant despite exponentially growing population: the number of nuclear armed countries, or even the number of countries, is not growing exponentially.

Contagious biological weapons could be this effective, detailed previously.  In general, we need weapons that autonomously spread themselves beyond the initial release in order to achieve killing 1% of the population regardless of the population size.  We might imagine some AI agent as well, though it's a bit hard to imagine an AI doing better than (engineered) biological microorganisms at spreading disease and killing because the biological organisms are already so good at it.

We interestingly assume that every technological civilization is vulnerable to contagious disease, defined broadly, and is therefore vulnerable to diseases that kill 1% of the population.  Every civilization must be somewhat monocultural in order to get along with each other enough to achieve technological progress.  Disease takes advantage of that monoculture to spread and kill.

There is nothing special about the 1% figures; it just needs to be constant proportions.

All this is inspired by Fermi paradox.  The reason we don't see aliens is because they all kill themselves off by the above-mentioned mechanism.  Strictly speaking, they don't need to kill themselves off; they only need to repeatedly and inevitably hurt themselves enough not to become space-faring galactic conquerers or colonizers.