Stop Casting Spell for Kill Shot?

[Totoy Tulog]

/run hpt=UnitHealth("target")/UnitHealthMax("target")
/run up=UnitPower("Player")
/run c=CastSpellByName
/run u=IsUsableSpell
/run if hpt <= 0.20 and u("Kill Shot")==1 then c("Kill Shot") end
/run if hpt >= 0.50 and u("Aimed Shot")==1 then c("Aimed Shot") end
/run if hpt >= 0.20 and u("Arcane Shot")==1 then c("Arcane Shot") end
/run if up <= 25 and u("Steady Shot")==1 then c("Steady Shot") end

Problem with my macro is that It wont Cancel Aimed Shot or Steady Shot Casting when Kill Shot is available at 20% HP or less.

Maybe there a way to do it.

I tried and didnt work.

/run if hpt <= 0.20 SpellStopCasting() and u("Kill Shot")==1 then c("Kill Shot") end