McAllister feared the pulsating tie at Rugby Park on Monday night was slipping away from the Paisley side when he conceded the spot-kick which allowed Kilmarnock to take a 3-2 lead in the first half of extra-time.
The 22-year-old was sure he had got a touch on the ball when challenging Brandon Haunstrup but referee Don Robertson immediately awarded the penalty.
St Mirren salvaged the situation with a contentious 119th minute spot-kick of their own, converted by Jamie McGrath, before prevailing 5-4 in the shoot-out when McAllister struck the winning blow.
“I definitely feel it was justice,” said McAllister. “I felt at the time that I had won the ball and then when I saw it back, it showed I did win the ball.
“I was positive I had touched it at the time but in your head you think you might not have. You get to see it after the game and it is much more positive if you have won the game and you get to look back on it but I definitely was sure I had won the ball. I felt I had won the ball and the referee got it wrong.
“That is football and luckily we got a penalty our way which got us back in the game. It was then nice to score the winning penalty in the shoot-out.
“I was confident I would score. We do penalties quite a lot in training. I am a positive thinker when it comes to penalties and I am quite happy to take one all the time. In my head, I am just going up to score. That is what I do in training every day so I am used to it.”
St Mirren now face League Cup winners St Johnstone in the semi-finals while Hibs take on Dundee United. With Celtic and Rangers both out of the tournament, McAllister is eager to seize the chance to make up for the disappointment of St Mirren’s defeat against Livingston in the last four of the League Cup in January.
“We look at that game and maybe we went about it the wrong way tactically but this is a chance now to go and get a win in another semi-final,” he said. “I don’t know the last time St Mirren were in two semi-finals in a year but we want to get to a final.
“As soon as you hear that Celtic and Rangers have gone out it gives you a boost of confidence going into the semi-final knowing that you won’t be facing one of them in the final. We go into every game positive and looking to win it. It won’t be easier to win the cup but the opposition is not as hard when Celtic and Rangers are out. But every game is tough so we just need to get on with it.
"St Johnstone have won one cup but why can’t we go and put them out? We have already put Rangers out in the League Cup. We have put the strongest opposition out this season so why can’t we go now and do it to them?”