Why didn't anyone ask the Deacon why he would drop Erin off at some park in the middle of the night? Seems rather odd not to follow up with that!
Who has access to the old cop revolver? Both John and Billy? My money would be on John.
Then we have the kid who thought the baby was his. He doesn't have the access to an old cop gun, though.
But why was Erin killed? Was she going to try to get money from John? Did John make Billy carry her down to the water after he killed her?