I think the most chilling part of that scene is how, aside from the removal of Sal's gun, everyone walks to the car in such a normal, calm tone...like the death of a long-time capo in the family was such a run of the mill thing.
Of course, another chilling component is the alacrity with which Tom participates in the murder. While the street thugs of the Corleone family could be expected to behave so nonchalantly, one might expect better of a well-educated person such as Tom.
