His death

[Crystal]

Nope, it's actually the opposite. Ventus knows that the x-blade will be forged if he fights Vanitas and is terrified of that weapon, that's why he refuses to fight him.
Vanitas counters by threatening to kill Terra and Aqua, actually forcing Ven to go to the Keyblade Graveyard.
Driven to a point of no choice left, Ven thinks of only one last possibility to prevent the creation of the x-blade => asking Terra and Aqua to kill him.

So he is telling Terra and Aqua to "put an end" to him?

Again, we don't know...we don't know if Ven's heart wasn't already complete again due to Sora's heart repairing it, nor do we know if he really needs this particular part back or if his heart can be healed another way.
The only clear thing about this is, that when Vanitas really got re-absorbed, Ven should have his darkness however tiny it was back and no longer a pure light heart.

You know, i thought when darkness re-absorbed to a heart.Then it will makes a complete heart. lol >.>