At the second press conference with Early there are times when's he's being open and times when he's not. Watch his body language, especially the hands. When someone asks if police know if she was targeted the body language changes and becomes defensive; this also happens when he talks about the cause of death, the additional tests, and the other investigation. That is also the point at which the FBI got involved, after the autopsy that morning.
Maybe if the killer targeted Vanessa it was really someone else he was angry at, someone specific who treated him very badly. Maybe he knew Vanessa in some distant way and objectified her but now that he has learned more about her from the press he does feel some guilt. She seems like such a giving, kind person, who would never intentionally hurt anyone, the last person this should happen to.
http://boston.cbslocal.com/2016/08/19/state-police-princeton-jogger-murder-fbi-vanessa-marcotte/