B737-800 NARROWLY AVOIDS COLLISION WITH ACFT MANEUVERING NEAR FINAL APCH COURSE TO RWY 8L AT ATL.

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ESTABLISHED ON FINAL RWY 8L ILS INBOUND. WE WERE ADVISED OF TFC BTWN US AND THE RWY. WE WERE INFORMED THE OTHER ACFT HAD US IN SIGHT AND WOULD REMAIN CLR OF OUR FLT PATH. 3 NM FROM RWY 8L; THE OTHER FIXED WING ACFT WAS OBSERVED TO BE ON A PARALLEL COURSE (092 DEGS) APPROX 150-200 FT TO OUR R; OF OUR INTENDED FLT PATH AND APPROX 100-200 FT AGL. AS WE CONTINUED OUR APCH; APPROX 400-500 FT AGL; THE OTHER ACFT APPEARED TO BE IN A SLIGHT R TURN AWAY FROM OUR FLT PATH BUT THE SITUATION DIDN'T SEEM RIGHT. THE OTHER ACFT WAS TOO CLOSE. THE OTHER ACFT WAS SLIGHTLY IN FRONT OF US AND 200 FT LOWER. HOW COULD HE SEE US? I DECIDED TO GO AROUND. SLIGHT CLB STARTED; MY FO INFORMED THE TWR OF OUR GAR; THEN STATED THAT THE OTHER ACFT APPEARED TO BE IN A L TURN FAR GREATER THAN 45 DEG ANGLE OF BANK TO 90 DEG ANGLE OF BANK -- 'RAZORS EDGE' INTO OUR FLT PATH AND PASSED UNDER US! I BELIEVE A PRECAUTIONARY GAR ON OUR PART WAS THE ONLY FACTOR THAT STOPPED A MIDAIR COLLISION. THE FIX: NEVER ALLOW FIXED WING ACFT TO ENTER THE APCH COURSE OF AN ACTIVE RWY. CLOSE THE RWY. DON'T CLR COMMERCIAL ACFT TO LAND IN THIS DANGEROUS CONDITION. THIS SMALL ACFT WAS IN THE WRONG PLACE AT THE WRONG TIME AND HEADING IN THE WRONG DIRECTION TO AVOID US. THERE WOULD HAVE BEEN 'NO' VERT SEPARATION IF I DIDN'T GO AROUND. IN MY 28 YRS OF FLYING; I HAVEN'T SEEN ANYTHING LIKE THIS!