Is the flag being set in a dialog? Maybe wait til the dialog is ended first.

Code
IF
DialogEnded("dialogName", _)
AND
DB_IsPlayer(_Player)
AND
ObjectGetFlag(_Player, "Eldithas_Fights_Player", 1)
THEN
DisplayText(_Player, "Flag Set");
ProcMakeNPCHostile(_Player, CHARACTERGUID_Eldithas_Histerix_3347b77a-4b37-40e7-9e36-60caef04cbe8);

Could use
Code
SetFaction(CHARACTERGUID_Eldithas_Histerix_3347b77a-4b37-40e7-9e36-60caef04cbe8,"Evil NPC");
to make them hostile too.