To DotA players...

This post is about Kill Rights in DotA...
I wish to know everyone's opinion on who should get the kill in the following scenario:

INT hero X and STR hero Y are allies, fighting the sole enemy Z
X engaged Z first and used 2 skills on Z and fought with Z 1 on 1, Y came later halfway through the battle and hit Z twice, Y did not use any skills nor take any damage while X took all the damage. When Z was about to die, a Morphling just waveformed in and killed Z.
X said Morphling KSed him while Y said Morphling KSed him.
X argued that the person who engaged the enemy first and did most of the fighting should get the kill. However, Y argued that the person who is able to (which means he can, but he did not) deal the most damage should get the kill, regardless of the other factors.

So, what do you think? Does X deserve the kill, or Y? Please vote at the poll section on the left. Thank you.