The whole process is population control based on alien morality.
Aliens observing earth determined that humanity was badly overpopulated, and the only way to avert disaster was to eliminate 98% of the human population. They decide that the best way to go about it is to let humans decide which 2% deserves to survive. In their alien thought processes, putting 50 people together in a room and letting them vote on who dies seems like the most straightforward way to do it. Since this process would involve over 100 million cycles, they automated the process completely (which would explain why we never see or hear any of the aliens), and set their ships to pick up humans, churn them through the process, and return the survivors, confident that this was the one person in fifty who their fellow humans have selected this person as being the best specimen of humanity in that group. Because their mindset doesn't comprehend social manipulation or the drive to protect the innocent, it never occurs to them that the process would yield a mix of the young and helpless and the completely amoral.The circle is programmed to target pregnant women twice as a condemnation measure.
In the event that humans are not murdering each other For the Evulz and some sort of logical judgment is being passed on humanity, the pregnant woman of the group would always be assigned a spot in the front ring, so that if/when she is killed, she will collapse backwards and fall on the rear ring. By forcing the participants to see an unborn fetus go through its own vote, the aliens are attempting to call the participants' moral fiber to the fore, a nonverbal You Bastard!. The Silent Man accepted he was going to die, and refused to participate as a non-compliant protest.
Perhaps he was already dying, or accepted it during the course of the game, but either way, he decided not to play as a final act of defiance.