As Lestrade crept closer to the building, he had to force himself to stay calm. It would be no use to neither of them if he started to panic now. Which, as reasonable as it was, was probably one of the hardest things he had ever needed to do. And then he heard a yelling voice and though he could not be sure, because it was echoing in the big empty building and Greg was still outside, just barely having reached the wall, but he had a feeling it had been Sherlocks voice. He gritted his teeth and hurried on, his hand still hovering over his gun, but not drawing it yet.
That changed the second he heard a gunshot from inside. Then another. Now, Greg could no longer hold back the panic making his heart thump wildly as if trying to beat ut of his chest and forcing him to take deep breaths and make an effort to concentrate properly and hear anything beyond the rush of blood in his ears and feel something else that the hard beat of his heart. He reached a door and carefully tried for the handle. It was open. He stopped, undoing the safety of his gun before slowly opening the door and peering inside. It was mostly dark and he couldn't see much, but there seemed to be noone close by, so he slipped inside, hurrying to close the door behind himself to avoid raising attention he could not use.
His gun held up, ready to shoot at anyone that proved to be a danger, he crept along the wall, keeping in the shadows as his eyes adjusted to the lack of light in the old building. He glanced around. He had gotten in at an end of the warehouse, where there were still tal shelfs filled with boxes and casks, hopefully hiding him from view until he could get an impression of his surroundings. He looked around when, rather close-by, he heard a pained groan. Greg froze for a second, stealthily creeping along the shelfs to where he had thought the sound to have come from and stopped at the edge. 
From there, he could see the rest of the building was mostly empty. And not too far from himself, he could make out two people. One standing in front of the other one, who was sitting. The latter was defiently Sherlock. And passed out if the way his body had slumped over was any indication. So that left the man standing to be MOriarty. Greg stopped and watched. He wasn't sure what to do. He could barely, if at all, handle Sherlock. How was he to handle someone just as smart but decidedly insane. A whiff of something metallic hit him, raising the panic he had just-so barely managed to fight down after hearing the gunshots. Was he too late? Had Moriarty already..? 
Greg swallowed hard, raised his gun and stepped into the open space, his weapon and eyes fixed solily on the madman. "I am here. Now let him go." he said, proud to hear that his voice sounded steady and strong.