Tomas thinks about a planet where people from earth would go to be born again. They would be born with all the knowledge and experience from their life on earth, and then they would go on to a third planet, and a fourth, and so on. That was eternal return to Tomas, and that was how he thought of optimism and pessimism. Optimism was thinking that life on planet number five would be peaceful; pessimism was thinking it would still be bloody.
The narrator implied earlier through his reference to Robespierre and the French Revolution that humankind’s atrocities are proof of the nonexistence of eternal return, but here Tomas suggests this isn’t necessarily true. Thinking that life on planet number five would still be bloody implies that humankind would continue to commit atrocities knowing full well what they were doing was wrong, which suggests it is human nature to behave in such unspeakable ways—whether or not life repeats.