“This is not one more text message that basically put him over the edge,” said Neumeister, testifying for the defense. “This text message was a game changer.”

Treacy replied to Ratley’s message with a threat to “strangle the life” out of her. He also sent text messages to friends telling them he was going to jail for murder, and even explained to one friend how he would do it.

“Snap her neck and stomp her skull,” he wrote.

Treacy also appeared to plan his affairs, leaving a weight set to one friend and telling him to give a ring to a girlfriend.

After searching online for directions to the middle school — Treacy was a freshman at Deerfield Beach High School at the time — he put on his dead brother’s blue jeans, a black T-shirt, black gloves, and steel-toed boots, and rode his bicycle to find Ratley.

The extent of Treacy’s planning shows that he was aware of his actions and that they were wrong, said Steiner, testifying for the prosecution.

“A whole sequence of messages, in psychiatric parlance, would be called instrumental behavior,” he said. “What it essentially means is he’s planning out — not really effectively and not very smoothly, and certainly in an agitated and angry fashion — what it is that he’s going to do.

“If you’re in a profound dissociative state,” Steiner said, “you don’t do that. You’re not capable of being instrumental and putting this much planning into the steps.”

But Steiner added later that Treacy told him during a psychiatric evaluation in 2011 that he never meant to seriously injure Ratley — and that he personally doubted Treacy intended to commit murder, given the boy’s lack of a violent past.

“The extent to which he was going to proceed with [the plans announced in text messages] could be anything,” Steiner said. “It turned out pretty badly.”

Closing arguments in the trial could come Friday.