Yes. And..because he wanted to. Personally, I think he likes the power of getting rid of her himself and not getting "caught". Being "smart" enough to not get caught and smart enough to put her somewhere no one can find her...to be smarter and to deprive her family from knowing where she is or what happened to her...to control all knowledge.
But we all know he was stupid enough to leave blood and run all over the place with trash bags.
Nevertheless, he is not in jail, but at home with his HO and jogging freely in front of the cameras while JDs family can only wonder where she might be.
oh and he has a slick attorney...for free.