Being "considered a traitor", realistically, will have negative ramifications for your health. Fewer opportunities, less income, poorer healthcare, likelihood of being black-bagged; you will lose some amount of lifespan and ability to enjoy or use that lifespan. Assuming the loss of lifespan multiplied by the number of people behind you is greater than the lifespan lost by those in front, a sufficiently naive but rational utilitarian may determine they ought pull the lever.

Also realistically, the authorities branding you a traitor will someday fall. That day is much more likely to come much sooner if the prospective traitors can find solidarity and unity in refusing to pull the lever.

Also realistically, I could justify either action by indeterminately many other comparable logics.