User blog comment:Jkphantom9/Volume 3 Episode 9 Preview/@comment-26389992-20160114030502/@comment-25396609-20160114052059

The phrasing was neutral as to who was involved, we only know for certain that Mercury and Marcus were combatants, not who they were fighting or why.

Assassins make a lot of enemies, it is possible someone attacked the home, they won but the house went up in flames, both Marcus and Mercury were injured, Mercury dragged Marcus out and he expired afterwards.