The home side had a Dylan McGowan goal ruled out after the assistant flagged for a foul by Jason Holt on United goalkeeper Radoslaw Cierzniak.
Locke, whose team are now eight points adrift of St Mirren, said: "I felt we should have taken all three points.
"I've seen the goal there on the laptop and it's a goal. It's another sore one to take because it's another big decision that's gone against us.
"The linesman disallowed the goal, I had a word at half-time and he said there was a push.
"Unfortunately we've just got to accept that and move on. From where I was it looked like the keeper just misjudged the throw.
"We've had a couple of bad decisions against us, we've got to get on with it. We can't let it affect us."
United had a goal disallowed of their own when the impressive Nadir Ciftci's effort was chalked off for a foul by David Goodwillie.
United manager Jackie McNamara said: "I thought it was a free-kick (for Hearts' disallowed goal).
"I watched it back. I know it's soft but it's still a free-kick. The lad went into him first as he's thrown the ball and he's went in again to put him back towards to his goal.
"It's a foul. Nadir's one for the foul against Goody, I thought he just used his body there.
"I thought there was not much in that. I know Lockie is a bit frustrated with their goal but it was a foul."
Locke's disappointment was tempered by the return of Ryan Stevenson, and the Hearts manager added: "I was delighted, he is a big player, a very important player for us.
"He is great in the dressing room as well so it's great to get him back on the pitch again."