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

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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.