The question about the judge, Brian Gain of King County Superior Court, is addressed here—where you can also watch a video in which Judge Gain explains his reasoning. As for the question about the prosecutor, here is what the Seattle Times has reported:

Deputy King County Prosecutor Zac Hostetter asked Gain to remand Kalebu into custody, citing the University Place homicides [in which Kalebu's aunt died in a suspicious arson after kicking Kalebu out of her place], concerns over Kalebu's mental instability and the protection order his aunt had filed against him before her death. But Gain denied the motion.

And, in a different Times story:

Deputy King County Prosecutor Zac Hostetter asked Gain to remand Kalebu into custody "based on concerns of mental instability and Rachel Kalebu's protection order filed just prior to the arson," according to documents filed in King County Superior Court.

Hostetter also cited the Pierce County homicide and arson investigation, and noted that Kalebu had missed an earlier court date.

Gain denied the motion.

This could lead one to imagine that Hostetter came into court with prosecutorial guns blazing, suggesting to Judge Gain that it was likely Kalebu had just killed his aunt and arguing strenuously that Kalebu be held because of that potential murder and his past mental instability.

But watch the tape.

Hostetter actually did not speak to the judge about the aunt's protection order against Kalebu. And, when Hostetter brought up the recent arson, he pointed out that while Kalebu had been questioned as part of the arson investigation, authorities had then released him. "He is not being held on that," Hostetter said.

As for the concerns about Kalebu's mental instability, Hostetter pointed out that the judge had previously ordered Kalebu to comply with the recommendations of his mental health treatment providers—to take his medication as prescribed and so on—but added: "I don't have any indications, your honor, that he has violated that in any way."

The argument Hostetter seemed to be making was that Kalebu's recent interactions with the law—which, by Hostetter's own account, had resulted in no charges against Kalebu—were themselves an indication of instability and a potential violation of another part of the judge's previous order: that Kalebu have no law violations.

But Kalebu hadn't actually had any law violations. Plus, Hostetter was telling the judge that there wasn't evidence that Kalebu was off his meds. And the tone of the hearing was far more procedural than "Put this guy in custody now!"

Which is not to say that Hostetter didn't try to have Kalebu held. He clearly did. But the law is the law, the evidence Hostetter had at the time was the evidence Hostetter had at the time, and Judge Gain's ruling was Judge Gain's ruling.

“Look, hindsight’s 20-20," Ian Goodhew, deputy chief of staff at the King County Prosecutor's office, told me recently. "Zach certainly didn’t get up there and argue, by any stretch, 'Hey, we have concerns that this guy’s going to go out and commit a brutal murder' ... [But] I think Zach took the appropriate approach under the rule [concerning release of criminal suspects]. I believe all the points were appropriately made. But what’s done is done.”

Goodhew also made it point to praise Judge Gain.

“Judge Gain is a long-term, highly respected member of the bench," he said.