This was a valuable victory for Kilmarnock and Derek McInnes, who made five changes to his team in response to the 5-1 hammering by St Mirren last week - a fourth defeat in five games which set alarm bells ringing.
Such was the tension created after that loss, a smattering of fans held up a banner as the teams emerged at Rugby Park which read: 'Relegation Pending'.
But after a scrappy start it was Kilmarnock who got the crucial breakthrough.
They won the ball on the edge of the Motherwell box, Bobby Wales lashed a low shot which was tipped on to a post by Ellery Balcombe, and when the visitors panicked, they pounced.
Armstrong laid it across goal, and goalkeeper Ellery Balcombe parried the ball to Murray, who slotted home.
The red card for Balmer was a gift, but one Kilmarnock gleefully accepted as they pressed home their advantage.
The only disappointment is they did not score more goals, and that Dundee below them also registered a victory in the battle to avoid the drop.
The return of Stuart Findlay to the defence and captain and striker Kyle Vassell to the bench are big boosts for McInnes for the run in.