Mac has the best possible description of any character in this book. Steinbeck calls him "a large man, with the face of a scholarly prize-fighter" (14). This strange duality sums up Mac's character pretty well. He's got just enough practical philosophy under his belt to know how to manipulate everybody around him—and he's also got the fists to back that up.
Mac's utter practicality makes him the perfect Party man—even if it makes him a questionable human being. Perhaps his most dubious adventure in the book is his delivery of Lisa's baby. While he probably does do a better job than the filthy old woman in the tent would have, he has zero qualifications. Everything he's said about them is a lie.
He explains to Jim this need to "use" everything around him, especially where the cause is concerned—even if it destroys others:
"We've got to use whatever material comes to us. That was a lucky break. We simply had to take it. 'Course, it was nice to help the girl, but hell, even if it killed her—we've got to use anything." (48)
It's a chilling philosophy of use, one that neither cares for nor respects the emotional life of human beings. Mac has no problem sacrificing anyone for the greater good. What if Lisa had died? No biggie for Mac. He could have used her death to incite the men somehow. It would all have been for the cause. Doesn't matter if Lisa herself has no say in it.
Mac's especially hardened to individual need because he knows of the great inequality and mass suffering around the country—even around the world. For a guy who thinks that Doc Burton is too abstract, Mac grounds himself in some pretty theoretical concerns. It's this solidarity with people he doesn't even know that makes the men around him a little bit suspicious. They constantly ask Mac, "What's in it for you?"
That's a mighty good question to ask, especially since Mac seems to be so determined to use everything close to hand—BFFs included (Joy's and Jim's murdered corpses, anyone?)—to further the cause.
Although Mac is responsible for some of Jim's more callous behavior, he does have some feelings to spare for those around him who suffer. When Anderson loses his barn, Mac feels bad for him. He knows that Anderson isn't 100% on their side, yet he's still suffering the loss of everything he valued.
Despite his total utilitarianism, Mac is kind of blown away by Jim's heartlessness. Unlike Mac, Jim doesn't give a hoot for Anderson's barn. He believes that Anderson should be glad to sacrifice something so small when he and Mac are willing to give up their entire lives. Mac actually fears the transformed Jim—which is impressive for a guy willing to manipulate everyone for his agenda.
Doc Burton recognizes this duality in Mac's personality. Mac is totally callous and ultra-practical most of the time—but he can mix a little human emotion in with this Party man personality. Here's a guy who's able simultaneously to mourn the death of Joy and calculate how to display the corpse for maximum effect.
Mac can also talk about a feeling of solidarity and concern with workers all over the world while hoping that federal troops are sent out to kill some of them (because it will be good for publicity). His whole personality is a wonder to Doc:
"'Mac,' he said, 'you're the craziest mess of cruelty and hausfrau sentimentality, of clear-vision and rose-colored glasses I ever saw. I don't know how you manage to be all of them at once.'" (163)
Mac doesn't care for Doc's theories at all—especially the bit about group-man. Mac likes to believe that he stands out from the pack, that he's a leader of men who will change society through non-conformity (a.k.a. "sticking it to the man"). But Doc Burton spoils that theory by explaining that Mac could be a leader but still be brainwashed by "group psychology":
"You might be an effect as well as a cause, Mac. You might be an expression of group-man, a cell endowed with a special function, like an eye cell, drawing your force from group-man, and at the same time directing him, like an eye. Your eye both takes orders from and gives orders to your brain." (114)
In other words, if you ask Doc, Mac's still part of a system that causes misery. He just has a different goal.
Despite his frequent lack of human feeling, we do get glimpses of Mac's softer side. He frets about Anderson and Al, who get hurt because of him. He nearly vomits when he has to beat the boy sniper. And he certainly has all the feels for Jim, for whom he feels friendship and responsibility.
But we can only see this feeling of responsibility come crashing down on Mac when the strike begins to fail. His lack of "professional success" forces him to think about his role in more personal catastrophes, like Doc's disappearance:
"It's my fault for letting him start over there alone, all my fault. London's doing everything he can. I forget things. I'm getting a weight on me, Jim. Anderson's barn's right on top of me." (209)
Mac's human qualities also make him the perfect target for Jim's zeal. Jim can't understand Mac's sentimentality in the face of the "bigger picture." Jim certainly doesn't care for Mac's emotional attachments or his attempts to protect him from bodily harm. In a sense, Jim really schools Mac on how to behave without humanity—and then reaps the dubious rewards of it.
As Jim lies face down in the earth outside the camp, Mac could fall apart over the death of his new friend. We know he has the capacity for it. Instead, he follows Jim's earlier advice to "use him till there's nothing left" and posts the mangled body in a very public place. It looks like deep down inside, Mac really is the ultimate Party man.