Wrong aproach.
The one that caused the damge is retrieved in _this select 1 by the code that is executed when "Hit" event happens.
So, _this select 1 might be Bob or Peter, etc.
what I mean is that addScore adds score locally to the passed unit, and Hit EH is executed locally in the machine where the damaged object is local.
So, for example you have Bob, who is a player, and Bob hits a building, which is local to the server. The code of the EH is executed in the server, and in the server Bobs gets -10 points. Now that modification of the score might not been broadcasted to Bob's computer, so might be Bob cannot see he now has 10 points less. This is something you should try online.