Coworkers Watched Chris Watts 'Get More and More Unhappy with His Life' Before Family Murders

In the summer of 2018, Chris Watts was working as an oil field operator at Andarko Petroleum in Frederick, Colo. His pregnant wife, Shanann, sold weight loss products from home where she took care of the couple's two young daughters.

From the outside, it looked like Watts was living a life of suburban bliss — but those who worked closely with him began to see a difference in the affable father who kept pictures of his daughter on his phone.

"Things changed; he changed," Brian Spence, a contractor who worked with Watts for several years, tells PEOPLE. "I watched him get more and more unhappy with his life. He went from being a friendly guy to being withdrawn and angry."

"I saw him lose his temper over little workplace annoyances," Spence continues. "He had an irritable side with the contractors. He could be combative."

It was unclear what had caused the sudden change, but Spence noted that Watts was no longer in a hurry to rush home every day after work.

"He did a lot more happy hours with us contractors," he says. "After work we'd go get beers and he'd come and have one with us, and he was always one of the last ones to leave. We're all family men and had the get home to our kids, so we'd drink from 6 to 7 and then go. He'd want to stay past 8, if anyone was willing to stay."

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But despite Watts' behavior, Spence says he didn't suspect that he would eventually kill his wife and two daughters before stashing their bodies in the very oil field where he worked.

On August 13, 2018, Watts strangled Shanann in their Colorado home, then he drove her body to a job site at the oil company. He buried his wife's body and then smothered his daughters, Bella, 4, and Celeste, 3.

"That, I didn't see coming," says Spence. "I knew he was unhappy, but he's not the first unhappy guy in the history of the world, and most of them don't kill their wives."

Another thing that few of Watts' collegues knew: He was having an affair with a co-worker who thought he was already separated.

"He didn't talk about his affair," says Spence. "But he was always like 'this or that chick wants to bang me.' But I didn't know it was actually happening. I thought it was just talk."

Looking back, Spence says that he wishes he had picked up on the warning signs.

"He was obviously in pain," he says. "But I didn't know it was going to end that way. If I had any idea, I would've pulled him aside and said something. But he hid it. No one knew."