U can try 2 methods I checked on my own i9505 (I have 4.4.2 version with old no Knox bootloader):
1. Use TWRP or PhilZ Touch custom recovery. After U exit from that custom recovery it will ask about root your phone. Then U can download SuperSU from market. When U launch SuperSU it will ask to disable Knox sw.
2. Or run
DE-SAMSUNGNIZER_KNOX_REMOVAL_SCRIPT_V1.0 from one of that custom recoveries and it will disable Knox sw.
BR