"They don't have fun there. There's nothing fun about it. There's nothing happy about it," he told Eric Branch of the San Francisco Chronicle. "I didn't enjoy any of my time there, you know what I'm saying? It made me for the first time in my life think about not playing football because I hated it that much."
Marsh added that he felt the Patriots miscast him as a linebacker and all but requested to be cut by the team in November.
"I confronted [Bill Belichick] about all the things that were going on," he said. "I won't get into detail, but it was B.S. things they were doing. I just wasn't a fan. And so I, basically, without asking to get cut, I kind of asked to get cut. ... I had confidence that I would have an opportunity elsewhere and I would take advantage of it."
Patriots fans apparently did not take kindly to those comments:
"I just want to address all you Patriots' fans who don't like this new article," Marsh said in an Instagram story. "I'm sorry to hurt your feelings, it seems to be breaking your hearts."
"But if you can't handle the truth, stay off my page. Don't read articles," he continued. "That's how I felt. That's how I still feel. And I'm grateful to be away from there, grateful to be where I'm at. And it helped me be grateful for where I was."
Marsh, 25, was drafted in the fourth round by the Seattle Seahawks in the 2014 NFL draft but didn't see much playing time, registering three sacks in his first three seasons with the team. The Seahawks then traded him to the Patriots for a fifth- and seventh-round pick in September.
In New England, Marsh registered 19 tackles and a sack in nine games, but he felt he was wrongly utilized as a "coverage" linebacker.
"They asked me to do a bunch of stuff that I had never done: Covering running backs and receivers and basically almost never rushing the passer, which is what I did in playing defensive line," he told Branch.
The Patriots ultimately released him in November, and he was picked up by the 49ers on waivers. He was a more natural fit in San Francisco, posting 11 tackles, two sacks and two forced fumbles in six games, and he appears likely to have an important role off the edge for the team in 2018.
He certainly wants to have a future in San Francisco.
"Hopefully I'll be a Niner forever," he said.