"HE IS MAR" -- MAR on Wikipedia says:  

"Mar or Mor (as pronounced respectively in eastern and western dialects, from Syriac: Mār(y), written with a silent final yodh) is a title of respect in Syriac, literally meaning 'my lord'. It is given to allsaints and is also used before Christian name of bishops.[1]"

MEANING --- "HE IS MY LORD"

I think it's not meant to be a clue for us about a specific person, per se, as it wasn't written by a victim, and because MAR and MOR are used so widely throughout the show for various character names and places -- I think the MAR reference is about an overall theme and that "He IS Mar" was written by RJ or an RJ helper, and it is a message based on RJ's crazy, sadist thinking.  This statement must stand for something he believes in or considers himself to be -- which may be God, or the Devil, or in service of...

MAR & MOR also both can mean THE SEA and RED or to HARM.  Numerous references are made, including:

DuMAR Tanner, Lorelei MARtins, Ellis MARs, MARta & Virgil ROMan, MORgan Tolliver, Towlen MORgan, MARion Hicks, David MARtin, MARcia Wallace, Santa MARta, MARquesa, MAR Verde Real Estate Corp., RoseMARy Tennant, MARiska Kolpecki, Villa MARsante, MARsante Security --- and I'm sure many more... 

In the FIRST SEASON, there were some repeating themes: [besides MAR and MOR]:

1.  A REDHEADED CHARACTER appears in almost every episode, as well as names that mean RED

2.  The murders the CBI solve are generally about:  MONEY, SEX, REVENGE or ACCIDENTAL; but RED JOHN is entirely different.  His reasons are sicker and even more sinister, likely apart of some demented religiosity he's got going about himself.

3.  HYPNOSIS is featured and used by Jane, and mentioned often --- one episode centers around it, too -- a tool that is probably greatly utilized by RJ.

4.  CLOWNS:  PJ is called a clown in several episodes; and then there's the smiley face and carney background

5.  BOATS/MARINAS/the OCEAN/MOBY DICK THEMES -- a guy is caught when Jane spots a copy of MB, realizing he's a smart guy who is hiding behind a "slow" persona.

6.  TWINS or a CHILD that was kept secret

7.  GOLF:  FIRST episode was about golfers/Tagliaferro & Tolliver both mean "cut iron" or "iron cutters" --- which could also mean a golf club