MAZE RUNNER SPOILERS!!
Newt being the one to get infected and killed was heartbreaking but at the same time, it was absolutely necessary for the plot.
Newt was a relatively stable character. He was calm and collected, and more often than not, the voice of reason. He was the quite literally the glue of the group (not WICKED's meaning). It had to be him who got infected, so readers could visualise what the Flare does to people. You could be the most level-headed person to exist, but the disease is so deadly that it would turn you into an unrecognisable monster. For example, if Dashner had chosen to write Minho as the one who got infected, the drastic effect the disease has on personality wouldn't have come through, since he's already hot-headed.
Up until Newt gets infected, we see the Cranks as vile, zombie-like monsters. Newt turning into one of them humanises them. It allows us to view them as people who had loving families and a life before the Flare, because we built an attachment to Newt over the trilogy, only for it to be ripped away. This allows us to visualise and empathise with the "Cranks".
Newt's infection and eventual demise are a turning point for Thomas. It's the final push his character needs to take down WICKED. Where Chuck's death was symbolic of the termination of innocence, Newt's is symbolic of the majority of humanity.


















