What UreDe4D said is a good idea, using player groups.
I'll explain it to you a bit more.
1) Create a PlayerGroup Variable with an array. (I named it Alliance.)
Inside Alliance[1] we have: Players 1 to 9.
Inside Alliance[2] we have: Player 10.
I want you to add these to their groups first and make them treath each other allies and enemies. (Look at the trigger below.)

Initialization
Now you want to put people in Alliance[1] to Alliance[2] when a unit fo them dies right?
If so then add this trigger aswell.

Unit Dies
This should do the trick, also I used a Boolean Condition to check if the owner was inside Alliance[1]