I am not sure what you mean by units in grp also executing the script? Are they also East, if so they will already, and be the killer.
Anyway, is this what you want to do:
;kill1.sqs
_killer = _this select 0
_units = units grp1
#mainloop
_i=0
#loop
_unit = _units select _i
if (_killer distance _unit <5) then {goto"foundhim"}
_i = _i + 1
if (_i < count _units) then {goto"loop"}
goto"mainloop"
#foundhim
.
.
.
_unit will then be one of the guys that is < 5 from _killer.