Say that Viola the Victim and Percy the Perpetrator have a dispute, but don't want to go to court. However, William witnesses everything and wants to take Percy to court over what he did. Can William reasonably do this?
In the extreme case where the crime is murder and William is the government, then I'm pretty sure this happens. But what if the crime is domestic violence, and William is just a stalker who has the hots for Viola? Or Viola is an old lady with dementia, and William is a good samaritan who notices she's being abused?
Basically, can a private citizen with "no horse in the race" (i.e. a third-party) reasonably take someone to court over something?