Fred Casely (Dominic West)
Roxie is cheating on her husband with hottie Fred Casely. He's a lying slimeball with a wife and five kids left to fend for themselves after Roxie plugs him. Not that anyone cares.
What we're mostly shown about Fred is his terrible attitude. Before Roxie shoots him, he admits that he never could have made her a star:
"Sugar, you were hot stuff. I woulda said anything to get a piece of that."
And then he pushes her. Eww, eww, eww.
Basically, as the "Cell Block Tango" says,
"If you'd have been there, if you'd have seen it […] you woulda done the same."
Because the movie doesn't show us anything nice about Fred, we can totally imagine that we woulda done the same.