The assassination move (entered by holding melee) is essentially only a taunt as melee from the back kills instantaneously and the assassination takes around ten seconds to complete, not even guaranteeing a kill since the "knife" doesn't hit immediately.
Additionally, a misbehaving controller can cause you to unintentionally enter the stance and lose otherwise guaranteed kills.
Halo 5 clearly has this setting, but I don't know about Reach in the Master Chief Collection.
Best Answer
Halo reach in the masterchief collection does not bring the setting to disable it as it did in halo 5. Reason halo 5 has it is because of mass tournaments that took place with fellow contenders request it to be a added option. I advise getting a new controller since it sounds to me you got sticky buttons.