When Stephen Marshall put a bullet through his head on a bus in Boston on Easter Sunday, he apparently took with him his reasons for killing two convicted sex offenders in Maine earlier that day.

But while Marshall’s particular motives have eluded police, some insight into the case could lie behind the heavily fortified walls of the New Hampshire State Prison in Concord, N.H.

It’s here where Lawrence Trant is serving time for attempted murder after setting fire to the homes of several sex offenders and stabbing another with a 14-inch kitchen knife on the street in front of his home in 2003.

Trant, 59, has never met Marshall, but – on some level – Trant said he knows the young Canadian man.

“I understand perfectly why he did it,” Trant said in a Thursday interview at the sprawling prison, where he will spend the next 10 to 30 years.

“[Marshall] felt that every child who was molested was an offense against him personally,” Trant continued, paraphrasing his own motives, he said, for his vigilante-style crimes. “He was just sick and tired of it.”

Trant not only understands the young Marshall, he said, but mourns his death far more than those of his victims.

“I don’t feel too much pain for the people he killed,” Trant said. “I don’t feel we have the right to do that, but it’s not a great loss to our society.”

Police say Marshall, 20, shot and killed two men, Harold Gray, 57, of Milo and William Elliott, 24, of Corinth, and stalked several other registered sex offenders after finding their home addresses on the state’s online registry.

Marshall had been visiting his father in Houlton, where detectives say he stole a truck and three handguns before committing the murders.

Marshall later killed himself when police cornered him on a bus outside Boston’s South Station.

Police had hoped that the laptop computer Marshall had with him on the bus would have provided clues as to why the introverted dishwasher would have set out on a killing spree.

However, after weeks of studying the laptop and the contents of two hard drives from Marshall’s home computers, police say they have no further insight into his motives.

“Unless something comes up unexpectedly before this investigation ends, it appears unlikely we’re ever going to be able to explain this kid’s actions,” Mane Department of Public Safety spokesman Stephen McCausland said Friday.

Marshall’s mother, Margaret Miles, told the Canadian Press she was disappointed that the investigation hasn’t yielded any answers, and would remember her son as “kindhearted and gentle.”

“I don’t know what was going through his head. He must have been holding something in, but he didn’t share it with us,” Miles said.

In some respects, Marshall’s case is similar not only to Trant’s, but to that of Michael Anthony Mullen of Bellingham, Wash.

In March, Mullen, 36, was sentenced to 44 years in prison for killing two convicted sex offenders he found on that state’s online registry. Mullen posed as an FBI agent to enter the victims’ homes before shooting them in the head.

Both Trant and Mullen had been in and out of prison before their violent outbursts, mostly for such offences as theft and check fraud and receiving stolen property. Marshall also had a run-in with the law as a juvenile when he used a gun to break up a fight at his father’s house when they lived in Idaho.Like Trant, Mullen said his violence against sex offenders was spawned by disgust for their crimes and his wish to protect children.

“Don’t get me wrong, I’m no saint,” Mullen wrote in a letter to a Seattle Times reporter shortly after the murders. “I’ve been haunted since childhood. I just don’t want other children to grow up confused, sad, scared.”

Trant said he was sexually abused as a child by a youth worker at his church. Mullen was abused by a neighbor while in elementary school, according to his brother.

There has been some speculation that Marshall also had been molested, but police have uncovered no evidence of any such abuse. During his Thursday interview, Trant, based on his own past, acknowledged it was a possibility.

“People don’t just take that kind of stand for no reason,” he said.

But to longtime prosecutor William Stokes, Marshall’s stand was nothing more than vigilantism at its worst.

Stokes, who heads the Maine Attorney General’s criminal division, said Friday he was “flabbergasted” by some of the comments he heard after the killings. Even people he respected, he said, would lament – perhaps jokingly – that Marshall didn’t kill more of the sex offenders on the state’s registry.

Had the Marshall case gone to trial, Stokes said that the prosecution would have faced a stiff challenge in overcoming the deep public disdain for the crimes committed by Marshall’s victims.

“Let’s not make this kid a hero,” Stokes said. “He was clearly a sick kid, but he’s still a murderer and that’s how he should be viewed.

“We may never know what made him tick, but there’s nothing positive about it,” Stokes said.

Trant doesn’t profess to know what made Marshall tick, and, upon reflection, he agrees with Stokes’ sentiments about vigilantism. Trant now spends much of his time writing to legislators asking for harsher penalties for sex offenders.

“I’m not ashamed of it, but I’m not proud of it,” Trant said of his crimes. “I did it for a reason. I’m not a monster. The monsters are the ones who victimize children.”