If the court is accepting that as Crittenden's justification, does that mean that the court accepts that Arenas made believable threats to Crittenden's life?
A few things here, from the legal end. The quote we have in the story is from Crittenden's attorney; it's probably spin. Now, the prosecution may have bought it -- which could suggest why he's getting to plead to a misdemeanor -- but it doesn't mean the judge did. In fact, it's likely that the judge didn't make any findings at all about Crittenden's state of mind or potential justification. He may or may not have explained what happened in open court -- called "allocution" -- but that doesn't mean the judge had to accept that story.
Also, there's no guarantee that Arenas' sentencing judge will be the same one who sentenced JC. The judge is under no obligation to accept the facts alleged by JC; in fact, the judge will probably never hear them on the record.
The big distinction is that Arenas plead guilty to a felony, and the prosecution will ask for a sentence of up to six months.