I'm pretty sure the appropriate answer is:
I can ruin the fun or make it dirty
A spoiler can 'ruin the fun' (like in giving away a movie twist) or 'make (something) dirty' by spoiling or ruining it.
I start with a hybrid form at atomic level
'SP hybridisation' is where the s orbital of an atom overlaps with one p orbital.
Something responsible for wars is right in the middle of me
Many wars have been fought over OIL.
If there is an emergency, you reach at my end
In a medical emergency you may end up in the Emergency Room, or ER.
You can try to hide your answer, but I shall reveal myself anyway
As is customary on PSE I have hidden my answer using a spoiler tag (>!) but it has still been revealed nonetheless!