With most of the gang members dead or in the middle of the process, Sebastian made his way down from his perch. He strolled through the now silent battlefield, humming a jaunty tune as he finished any survivors with quick thrusts of Lament.
"Now then," he said, looking around at the sea of bodies. "Time for the last act. Let's make sure our dear gang leaders draw all the wrong conclusions from this little drama."
Sebastian set about arranging bodies and planting false evidence. Not that he needed it, but there were some things that would fan the flames, surely.
At some point, he got to the body of the pyromancer he killed earlier, the one he based his illusion on, to start this mess.
He brought it close to the corpse of the Marauder he killed with his Flame Dart. That way, it would look like the guy got killed soon after the battle started.