Kilmarnock manager Allan Johnston rued his side's inability to build on a lead in
After going a goal behind, Killie moved in to a 2-1 lead before conceding four goals in the final half-hour.
"Unbelievable," reflected Johnston. "We got ourselves in a great position after scoring a fantastic second goal.
"I feel sorry for the players. They put in so much effort but it looks like they have been hammered."
Johnston felt a save by United goalkeeper Radoslaw Cierzniak proved to be the turning point of the game at Tannadice.
"We have a chance to make it 3-1 to probably put the game out of sight," said the Killie boss.
"The keeper makes a good save, they score the second goal and we shoot ourselves in the foot from there on.
"In reality, it wasn't really a hammering because we played a lot of good stuff. I think we can take a lot of heart from our performance.
"There were some positive parts which we can take in to the rest of the season as we had a very young side out there."