"Tom is an accuser and Harry is the accused" would normally imply that Tom is one of several accusers, all of whom are accusing Harry of something. If Tom is the only accuser then normally it would be "Tom is the accuser and Harry is the accused". "Tom is an accuser" by itself could be talking about Tom's general behaviour/character, without implying that he is acting as one of a group of accusers.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.