Help me understand this last turn

[plz delete account]

I was playing against some warlock. I had ten cards left in my deck, the warlock had already started taking fatigue damage.
I had 25 health and no armor, and he had six health and no armor.
He plays some 10/10 mech from the new expansion,
plays a spell that allows him to cast his next spell with health instead of mana,
kills his minion... which somehow automatically killed me?
I'm terribly confused and don't know how this was a interaction that should have killed me.
Can anyone help me figure out this interaction?

[Raetary]

he played mecha'thun.

[caractacus]

Ive been playing this deck, it's an even warlock. Once you have Mecha'thun in hand you play the two mechs that reduce his cost by one thus reducing him to 8 mana. Then when your deck is empty you play him, the 2 mana card that lets you cast spells for health and the 4 mana card that destroys all minions and discards your hand enabling the deathrattle effect.