Narrative:

Both aircraft called me 10 mi west of airport. Small aircraft X called in first then small aircraft Y. Small aircraft Y pilot advised that he saw small aircraft X on initial call up and was passing small aircraft X. They were both assigned downwind entries to runway 25L. They later both checked in 2 mi southwest of dvt having each other in sight. As I was about to issue a sequence (there were 4 ahead in pattern) small aircraft X apparently sped up and cut off small aircraft Y forcing evasive action. There was personal chipping on frequency and later I had to issue go around instruction twice to small aircraft Y since he was following small aircraft X very closely all the way to the runway. This is a very disturbing situation, since the near midair collision filing by the small aircraft Y pilot appears to be a personal grudge against small aircraft X pilot who may have intentionally cut him off. The pilots obviously knew each other prior to the incident. When the small aircraft Y pilot came to the tower cabin attendant immediately after the incident he told me 'if I didn't have passenger on board, I would have rammed him. We don't need pilots with attitudes in the air, and I don't need my career placed in jeopardy because of irresponsible revenge acts by pilots.

Related reports:

Google
 

Original NASA ASRS Text

Title: SMA X AND Y CARELESS AND RECKLESS OP OF ACFT IN ATA HAD NMAC. VIOLATION OF FARS.

Narrative: BOTH ACFT CALLED ME 10 MI W OF ARPT. SMA X CALLED IN FIRST THEN SMA Y. SMA Y PLT ADVISED THAT HE SAW SMA X ON INITIAL CALL UP AND WAS PASSING SMA X. THEY WERE BOTH ASSIGNED DOWNWIND ENTRIES TO RWY 25L. THEY LATER BOTH CHKED IN 2 MI SW OF DVT HAVING EACH OTHER IN SIGHT. AS I WAS ABOUT TO ISSUE A SEQUENCE (THERE WERE 4 AHEAD IN PATTERN) SMA X APPARENTLY SPED UP AND CUT OFF SMA Y FORCING EVASIVE ACTION. THERE WAS PERSONAL CHIPPING ON FREQ AND LATER I HAD TO ISSUE GAR INSTRUCTION TWICE TO SMA Y SINCE HE WAS FOLLOWING SMA X VERY CLOSELY ALL THE WAY TO THE RWY. THIS IS A VERY DISTURBING SIT, SINCE THE NMAC FILING BY THE SMA Y PLT APPEARS TO BE A PERSONAL GRUDGE AGAINST SMA X PLT WHO MAY HAVE INTENTIONALLY CUT HIM OFF. THE PLTS OBVIOUSLY KNEW EACH OTHER PRIOR TO THE INCIDENT. WHEN THE SMA Y PLT CAME TO THE TWR CAB IMMEDIATELY AFTER THE INCIDENT HE TOLD ME 'IF I DIDN'T HAVE PAX ON BOARD, I WOULD HAVE RAMMED HIM. WE DON'T NEED PLTS WITH ATTITUDES IN THE AIR, AND I DON'T NEED MY CAREER PLACED IN JEOPARDY BECAUSE OF IRRESPONSIBLE REVENGE ACTS BY PLTS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.