So..there might be something else going on which is causing that. I've recreated the trigger
Code:
Untitled Trigger 001
Events
Unit - A unit Begins casting an ability
Conditions
Actions
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Ability being cast) Equal to Missile
Then - Actions
Unit - Create 1 DummyMissile for (Owner of (Casting unit)) at (Position of (Casting unit)) facing (Target point of ability being cast)
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Unit-type of (Last created unit)) Equal to DummyMissile
Then - Actions
Unit - Order (Last created unit) to Move To (Target point of ability being cast)
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Distance between (Position of (Ordered unit)) and (Target point of issued order)) Equal to 0.00
Then - Actions
Unit - Create 1 Dummy for Neutral Hostile at (Target point of ability being cast) facing Default building facing degrees
Unit Group - Pick every unit in (Units within 512.00 of (Position of (Last created unit))) and do (Unit - Explode (Picked unit))
Unit - Kill (Ordered unit)
Else - Actions
Do nothing
Else - Actions
Do nothing
Else - Actions
Do nothing
and for me the missile moves to the spot, however there is no units that die and it does not create the dummy unit. From what i can see it matches however ive got no idea why yours would explode and mine does not. Test something for me, delete the action that explodes the units and try out the trigger. Do the unit still die afterwards?