Some of Bonnie's blood was still splattered on the floor, and Damon couldn't help but notice the mess that she had made when she forced him to drink her blood. Normally, he would be more than overzealous to indulge in such a feast, most especially the glimmering blood of a witch. But this time it was different, he had just saved Bonnie from death, death that was caused by a newly transitioned Elena Gilbert - who was turned by the oldest vampire in the history of time - Lord Nikolaus or simply Klaus. She completed her transformation by feeding on Bonnie - a willing blood donor, but since she was new, and the lustrous blood of a witch was too intoxicating for her, she lost control of herself and killed her in the process. Damon couldn't take that much of a risk, he had just pulled Bonnie's soul out of the realm of the dead, where the souls of her ancestors inexplicably held her captive. What Damon succeeded in doing was something even the illustrious and formidable Emily Bennett could not accomplish. Damon was drained, debilitated which is an inescapable repercussion of entering the realm of the dead, and when Bonnie realized this, she promptly offered her blood. Damon did not want to harm her by taking too much blood, he had refused her, and at that moment he felt like his brother, floundering to renounce his nature. He refused her many times, but they were both obstinate, but Bonnie - inexorable Bonnie was able to make the adamant Damon yield. She fed him his blood, fortunately, he managed to control himself, but barely. It was one of the most delectable meals he had, ironically, he didn't want it, because he knew that Elena would never have approved. Elena would only approve it if he was on the brink of death, but he wasn't, even if he collapsed in front of Bonnie, the truth is that he was fine. Emily Bennett assured him that he wouldn't die, but the strain would be great. And those are the side effects of going into the dimension of the dead.