Yes, mail delivery after dark would be putting mail carriers' lives at risk. Not only is there a risk of dangerous things happening - such as possible burglaries or gang-related activities, but driving at night in itself can be dangerous. Visibility is significantly decreased at night, so the risk of an accident is much higher.
Despite the fact the United States Postal Service might benefit financially from a decision to deliver mail after dark, the dangers associated with such a decision far outweigh any positives. Mail carriers would be placed in a deadly position if asked to carry mail on foot through densely populated areas. Poor lighting would greatly increase the threat of crime or violence. Likewise mail carriers who pull their vehicles to the side of the road on rural routes and greatly increasing the likelihood of a vehicular collision. Delivering the mail after dark clearly puts the carriers' lives at risk.
Our mail carrier works a busy street and I have seen numerous times during the day drivers that do not pay attention and swerve around them as they are pulling away from a mailbox. At night, I think those near accidents would increase greatly even though the taillights should be more visable at night. Drivers everywhere are in a hurry, and I would say at night this is even more true.
Personally, if I was a mail carrier, I would LOVE to deliver mail at night. I live in Arizona, and I feel bad for my mailman walking around my entire neighborhood up to each individual house delivering our mail in 110 degree weather. Now, I do live in a very urban neighborhood where crime does happen all the time. I also live in a state where it is very legal to where a gun on your hip unconcealed. So these men & women could very much protect themselves if provided necessary.