You can't really kill the voice of life. It's more like destroying a suit. But I guess if you want to count it as a death, sure, it's the first.
Actually Hydregion being murdered as a voice of life is WORSE than if he was a normal pokemon... Think about it, a GOD of the pokemon world, KILLED in physical form... I'm surprised it hasn't caused much controversy yet, especially when in FFVII Aerith had a similar death, not nearly as bad and everyone freaked out over it