Once we are dealing with scenarios where hundreds of millions of humans are dying, it becomes harder to rule out the possibility of some desperate political or military leader resorting to using nuclear weapons. From there it is a small step to full nuclear war and the deaths of billions of humans.

Perhaps the "optimistic" scenarios are ones in which the hundreds of millions of humans that die are located far away from any nuclear armed countries, or at least those countries aren't destabilised by the economic and migration consequences of those deaths. I can, unfortunately, imagine militaries being routinely used this century for wiping out the remaining civilian populations of failed states.

This thinking seems backwards. How can we learn to take a totally dead planet and make it living when we can't even figure out how to keep this one healthy?

Mars and Venus are, as far as every experiment has shown, dead as doornails. Using them as a testbed for experiments that are too dangerous to do on Earth without first practicing is a good use for them.