has been kept indoors in a room that gets sun from the south.
Junipers do not belong indoors.
Junipers should not be watered every day, they should be watered when needed.
What you have purchased is a young plant and currently not a bonsai.
You can cut that branch, it's dead (at least, the brittle top part is). As it seems the only one affected, it may just be mechanical damange and not much to worry about (yet).