if your script bits are within onplayerkilled.sqs then it should work fine. be aware that variables with an _underscore before them are local, so if you are using
_killer in another separate script, flashpoint won't know what it is. however, if you change it to
killer without the underscore in both onplayerkilled and your own script, it becomes a global variable and you can use it anywhere.
having said all that, i think onplayerkilled is the very last script called before the game ends (i could be wrong there), so you'd need to write your own onplayerkilled script - or change your existing one.
there's a tutorial on how to do it
here.