You dont pick the Sprite. You have to pick. Always.
System > Pick all > backgroundabc
System > Pick nth instance > backgroundabc ... 1
Sprite > Compare X > 400
Why pick all ? I wrote it there for the logic.
Index, or the expression Sprite.IID, or 'nth' in the condition, or sprite(n).Expression ...... , all stand for the index in the picklist. Always does.
If it is a root event, you dont need to explicitly use 'pick all', you can assume that they are all picked (like in in your case, when you dont explicitly pick em, you delete them all)
This will not work as you intend. (to illustrate IID)
Sprite > Compare X > 400 (picks all sprites with x > 400)
System > Pick nth instance > Sprite ... 1 (pick the second sprite in that pick list, but that picklist is not containing all sprites anymore, just does with X > 400)
Not sure though why you need IID.
Sprite > Compare X > 400
_____ destroy
Should just do fine.