WHY SATURN IS YOGA KARAKA FOR TARUS ASCENDANT

When natural malefic owns trine(9^th or 5th houses) then it will act like a malefic, if natural malefic owns kendra(4^th or 7^th or 10^th houses)then it will act like benific.For Taurus ascendant Saturn rules one malefic house and one kendra house  so it should  behave like both malefic and benefic and it should not be yoga karaka planet But why saturn is  yoga karaka for this ascendant??  because its first own good house( trine house which is 9^th) after that only bad house (10^th house rulership which  is generally bad for natural malefic ) and  it mostly  gives good results