If he had killed only the girl they were looking for, then wouldn't he have said, "there is no body"?
In "The Royal We", Torres explicitly compares the way Cal treats Loker to the process used to create a "disturbed personality". Cal demoted Loker to unpaid intern (and how is Loker supporting himself if he's always working for the Lightman Group but doesn't get paid for it?), rarely praises him, constantly mocks and belittles him, actively sabotages him on certain cases, punched him out for being kissed by Emily, and Loker can't do a thing about it if he wants to keep his position. And then comes "Sweet Sixteen", where Loker almost gets blown up, and you think the fallout of his constant Butt-Monkey status is finally going to be shown. But no, the episode focuses on Cal and Gillian (which believe me, I'm not complaining about, it was great backstory) and while Loker is snippy and bitchy and hacks Gillian's computer to get the truth out of her, nothing ever comes of the fact that his bosses almost got him killed and lied about it. He's LONG overdue for a meltdown (though "The Canary's Song implies that he's finally drank the Kool Aid and gotten Cal's respect). And the reason he hasn't practiced his "radical honesty" since Season One? He'll reveal just how angry and screwed up he's become.