Puppies. Who doesn't love themselves some puppies?
After the crew foolishly guns down a group of Vietnamese civilians whom they mistake for hostiles, they take a puppy from their boat. We see Lance bundling it up later on, carrying it as they venture further down the river.
The puppy's totally innocent. He just wants to chew on people's sandals and pee on things. He represents the innocent people that the guys have just shot to pieces. The puppy humanizes the whole slaughter scene; this is a family with a beloved little pet. It kills us to watch it.