Player Collide with Fast moving Rigidbody to cause Death?

I was wondering If anybody could help me;

I need to know how i would go about making a player die if it gets hit by a fast moving Rigidbody. I need the script and the collider trigger to be attached to the player, so i dont know how i could check that the velocity of the rigidbody is enough to kill the player.

// C# version
void OnCollisionEnter(Collision col)
{
if(col.relativeVelocity > someValue)// where “someValue” is a variable of your choosing.
{
killPlayer();// You’ll have to implement how the player will die, yourself, but just call the method(s) needed to kill them, here.
}
}

Or, if you use JavaScript

// JavaScript version
function OnCollisionEnter(collision : Collision)
{
     if(col.relativeVelocity > someValue)// where "someValue" is a variable of your choosing.
     {
          killPlayer();// You'll have to implement how the player will die, yourself, but just call the function(s) needed to kill them, here.
     }
}

Put one of these in a script on your character (choose whichever you like. Both do the same thing).