When Condit realized she is missing, his first instinct is to avoid involving himself in any way. He lies to her parents. He lies to police. Too late to go back and do the right thing now.
He really underestimated the vehemence with which the press would persue the story and how his silence contributed to that, or the degree to which people would punish him for not admitting the affair right away and telling all to the family and to police.
Who then took the time to try to bury her, which this type of predator doesn't usually do?