A former coworker of convicted murderer Chris Watts says that he was clearly unhappy in the weeks before the killed his pregnant wife and two young daughters and stashed their bodies on the oil field where he worked.

“Things changed; he changed,” Brian Spence, a contractor who worked with Watts for several years at Andarko Petroleum, 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 He went from being a friendly guy to being withdrawn and angry.”

Spence said Watts began to quickly lose his temper over little things, was “irritable” with contractors, and “could be combative.”

Spence said he didn’t know what caused the change, but suddenly Watts was more likely to go out with the contractors for beer after work, and he’d be the last one to leave. Still, Spence said, he didn’t think Watts would strangle his wife Shanann and smother his daughters, Bella, 4, and Celeste 3.

As CrimeOnline previously reported, Watts was convicted of strangling Shanann to death on August 13, 2018, then driving to the oil field — with his daughters and their mother’s body — where he buried her. Then he smothered the girls and stuffed their bodies into oil tanks. He pleaded guilty to the murders in November.

“That, I didn’t see coming,” said 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.”

Watts was having an affair with a coworker who thought he was separated from his wife, but few of his other coworkers knew about that.

“He didn’t talk about his affair,” said 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.”

Spence said it was obvious Watts was in pain and if he’d only known how bad things were going, “I would’ve pulled him aside and said something.”

“But he hid it,” Spence said. “No one knew.”

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[Featured image: Chris Watts and the family he murdered/Facebook]