Is it illegal to steal someone elses swag?

Presuming you mean someone's look or demeanor or how they carry themselves, it is next to impossible to legally copyright anything of that nature. For that matter copyright laws are such that protecting anything with copyright law is a time intensive and very expensive process, on the order of tens of thousands of dollars and months to years of time. For example, you have to show that you have not copied someone else before you can prove ownership of a style or design. So I would say no, it is not illegal to copy someone's swag.