Defense lawyers had sought to suppress the testimony of Maryland prison guards Joseph Stracke and Wayne Davis. The defense team argued that Malvo had already invoked his right to remain silent by the time he had spoken with the guards.
But Circuit Judge Jane Marum Roush ruled Tuesday that the testimony can be heard at trial.
"Although Malvo was in custody, and the prison guards were law-enforcement officers, Malvo initiated the conversations, and the guards did nothing deliberately to elicit any incriminating statements," Roush wrote.
Malvo, 18, and fellow suspect John Allen Muhammad, 42, have been charged with 13 shootings, including 10 deaths, during a three-week spree that terrorized the Washington area last fall. They are also suspected or charged in shootings in Alabama, Louisiana, Georgia, Arizona and Washington state.
The conversation in question occurred Oct. 26, two days after Malvo's arrest.
According to Stacke, he asked Malvo why he shot Iran Brown, 13, outside a Bowie, Md., middle school. "To make Chief Moose upset, to make him emotional so he wouldn't think straight, and it worked," Stracke said he was told by Malvo.
After the shooting, Police Chief Charles Moose had tears in his eyes while he pleaded with the killer to stop the violence. Brown was critically wounded but later recovered. Moose, who has since left the department, led the sniper investigation.
Malvo told Stracke that Muhammad shot two women in Alabama. The guard also said Malvo talked about cleaning up ghettos as a rationale for a killing in Louisiana.
Malvo also said he had initially planned to shoot a busload of children, but aborted that plan, Stracke testified. The reason was unclear.
A second guard, Wayne Davis, said Malvo claimed that he shot a senator on a golf course, that he shot a young black woman who answered a door, and that he had tracked down and shot a schoolmate who had been picking on him.
Davis said he didn't necessarily believe everything Malvo said.
"I was under the impression he was exaggerating to me," Davis said.
Malvo's lawyers said the array of claims calls into question the veracity of any of the alleged confessions.
"These sorts of comments, even if they're true, really fit what the defense has been saying about Malvo, which is that he brags about things, and he has some level of need to exaggerate his own importance and his own role in these crimes," said CBSNews.com Legal Analyst Andrew Cohen. "I don't know that the jury is going to buy that, but I think that is what the defense is going to be selling."
Malvo goes on trial Nov. 10 for the shooting death of FBI analyst Linda Franklin at a Home Depot store in Falls Church. He said he shot her because "she was just lazy, standing still," Stracke testified.