The man known only as Dex was a servant/slave of Morlun. He seemed to genuinely despise working for Morlun, even going so far as to request that he kill him, a request that Morlun respectfully declined.
When Spider-Man managed to severely weaken Morlun by forcing him to feed off of the radioactivity in his body, Dex saw his chance and shot Morlun and killed him. Despite his own turmoil over whether or not to allow Morlun to live, Spider-Man was not happy with what Dex did and promised to hurt him worse than Morlun ever could’ve if he saw him again.
As soon as Spider-Man was out of hearing range, Dex jubilantly began singing, “Ding-Dong! The Witch is Dead”.