I think he didn't care about leaving DNA because his original intent was either dumping the body in a lake, burning it far away from where people would see smoke or leaving it to be exposed to the elements.
The interesting thing is, she was carelessly left covered in his DNA, yet she was double bagged in garbage bags AND a tarp....he didn't want any fluids, fibers or hair left in or on the transporting vehicle. He didn't care about leaving his DNA on her, she was just an object that had to be disposed at that point....he was however careful that no DNA leaked out from her and on a car that he could be possibly tied to.
I don't think his original intent was to have her body left in a busy area where it would be quickly found. I think it was a mistake on his part or an accomplice. SOMEHOW that body accidentally went from vehicle to the asphalt in that neighborhood. That's probably why he was seen pacing back and forth because they didn't know where the body fell off and now people were starting to talk about a body being found. He got nervous and blurted out that it was Alanna before authorities told anyone it was her.