"Damon has been running from this for 50 years, and he is obviously still very emotionally messed up about it. He’s kept it from everybody out of shame, guilt, and fear. Elena had to pry it out of him because he believes that leaving Enzo, even under those circumstances is the worst thing he has ever done. I think in Damon’s mind, leaving Enzo is what solidified every bad thing everyone had said about him up until then, and it is a big part of what allows him to believe that he is the worst brother/vampire/person ever and undeserving of love and kindness now. And, the only way he could live with himself is by carrying out the revenge plan. Damon said he “had to carry out OUR revenge plan alone.” In his mind, he owed it to Enzo to keep on killing Whitmores because he left him there. Like, in a twisted way, that made it better for Damon and kept the guilt and self-loathing from overtaking him, both while he was off and now, even all humanitied up. I don’t think I’m explaining it well, but it’s such a fucked in the head thought process: “I have to kill Whitmores because to not kill them means that not only did I fail Enzo by not getting him out, but also by not even getting his revenge."