User blog comment:FairyRaven/Fairy Tail Chapter 467: Mother's Key, Review and Commentary/@comment-7418318-20160107102332/@comment-5303512-20160107133139

Marin has been always a softy for women and has unrestrained hatred for men, when Marin showed up to kill Brandish, it wasn't frustration but something else like he was struggling. It would just seem out of character if he didn't show his genuine angry face while doing it.

Plus with your reason, I wouldn't call Marin killing Brandish an assassination because he was killing her because he didn't like her. Assasination involves the killing of a particular target for political reasons or payment. The other Spriggan (namely Bradman) was just massacring everyone.

With Jacob, he is called the Assasin for a reason. Although what he could do we still have to see, but I am still leaning towards Jacob being responsible for Brandish joining Alvarez.