What would justify a citizens arrest in Washington state?

Answer 1

The same kind of justification that would prompt an arrest by a law enforcement officer. HOWEVER - check you local state's laws carefully. The laws for citizen's arrests vary GREATLY from state-to-state.

Answer 2

There are two possibilities in this case: Felony citizen's arrest and Misdemeanor citizen's arrest.

1) Commission of a felony when the arresting person has "probable cause" to believe the arrestee committed the felony. (Make sure it was committed, not about-to-be or in-progress.) This means you don't have to actually see the crime, you just have to have probable cause, what a REASONABLE person would believe to be true, that the arrestee committed the felony.

2) Alternatively, commission of a misdemeanor that is done a) in the presence of the arresting person AND b) constitutes a breach of the peace (Washington CPP: "A breach of peace ordinarily involves some actual or potential disturbance of public order and tranquillity, or conduct which tends to provoke or incite others to violence.") This can be a gray area, so don't just arrest somebody for shouting in the street.

In all cases, if time permits, it's better to call the police to arrest the individual, and be a "good witness" rather than a "citizen cop." Sometimes, though, time does not permit, and you must make a good faith effort. I suggest you look up "delegating arrest authority to police", "use of force in arrests", and what constitutes likely felonies and misdemeanors in Washington state. Good luck.