Yes, slaughterhouse workers suffer psychologically and become more violent.
The book Slaughterhouse (Gail A. Eisnitz, 1997) shares many anecdotes quoted directly from workers on slaughterhouse kill lines. Here's a quote from Tommy Vladak, a sticker for nine years at Morrell slaughterhouse plant in Sioux City.
"You're just putting in your time. And then it gets to a point where you're at a daydream stage. Where you can think about everything else and still do your job. You become emotionally dead. [...] And you get just as sadistic as the company itself. When I was sticking down there, I was a sadistic person."
"The worst part," he continued, "even worse than my accident, was what happened to my family life. I'd come home, my wife would ask me how my night went, and instead of being happy to see her I'd say, 'What the hell do you care?' We'd get into arguments about stupid things." [...] "My wife and I finally separated in early July," Vladak said, "about two weeks before I cut my face. She couldn't take the bitching any more. I'd blow up at the drop of a hat, come home every night and find something to complain about, and take my frustrations from work out on my family."
And here's a quote from Steve Jansson, another man who worked at the same slaughterhouse in Sioux city.
"The worst thing, worse than the physical danger, is the emotional toll. If you work in that stick pit for any period of time, you develop an attitude that lets you kill things but doesn't let you care. You may look a hog in the eye that's walking around down in the blood pit with you and think, God, that really isn't a bad-looking animal. You may want to pet it. Pigs down on the kill floor have come up and nuzzled me like a puppy. Two minutes later I had to kill them -- beat them to death with a pipe. I can't care."
"Sometimes I looked at people that way, too," he said. "I've had ideas of hanging my foreman upside down on the line and sticking him. I remember going into the office and telling the personnel man that I have no trouble pulling the trigger on a person--if you get in my face I'll blow you away. [...] Every sticker I know carries a gun, and every one of them would shoot you. Most stickers I know have been arrested for assault."
If you're not convinced by anecdotes, there are also studies like A Slaughterhouse Nightmare: Psychological Harm Suffered by Slaughterhouse Employees and the Possibility of Redress through Legal Reform.
This account invites a connection between the cruel nature of the slaughterhouse industry and the cruel actions of the slaughterhouse workers—having been instructed by their supervisors to rip off birds’ heads, the workers feel no remorse in tossing around dying birds “just for fun.”
In this country, we have a common understanding that taking pleasure in the cruel death of a helpless animal is an antisocial and potentially psychotic characteristic.26 The countless stories of slaughterhouse employees inflicting pain on animals “just for fun” indicate that the nature of the slaughterhouse work may have caused psychological damage to the employees, because the employees’ actions certainly rise to the level of abnormal cruelty that would cause concern among the general population.27
By studying the nature of psychological trauma, analyzing specific psychological mechanisms that would contribute to slaughterhouse workers’ trauma, and reviewing anecdotal evidence of slaughterhouse trauma, the remainder of this section will demonstrate that slaughterhouse workers likely suffer serious psychological trauma in their workplace.