Murder is very immature. In this story, Poirot demonstrates maturity because he is able to solve the murder of Ratchett. The immature characters are Rachett, who kidnapped and murdered a baby and then changed his identity, Colonel Arbuthnot and Pierre Michel.
Killing a man on a train was an immature move. When the train gets trapped in the snow, the murderer is stuck. Also, the murderers should have stopped to think about the fact that there are other passengers, because one of those passengers was the famous detective Hercule Poirot. They were doomed from the start.
Another instance of immaturity is in the incident that caused the baby’s death itself. The young nurse throws the baby off the train when she realizes she might get caught.
“The police were convinced that she had some knowledge of the crime. They refused to believe in her hysterical denials. Finally, in a fit of despairthe poor girl threw the baby from a window and was killed. It was proved afterwards that she had been absolutely innocent." (ch 8)
This is very sad indeed. The girl was innocent, so there was no need to toss the baby. The baby died for no reason, due to her immaturity.