An 'infringer' or one who infringes, is to break or transgress the law or rules.
A 'tortfeasor', or one who commits a tort, which is a LEGAL term for any civil wrongdoing for which an action for damages may be brought, commited either intentionally or through negligence
As far as I can make out: There is a subtle difference, in that a 'tortfeasor' is more of a legal term where compensation is wanted from the offender. An infringer is simple someone who breaks the law or rules.
I am not an expert in these matters - just some research into legal dictionaries!