My theory - the cop is a perv just like the killer
There is a gaping flaw in the character reaction in this film - the cop realises he is essentially being blackmailed by the killer but instead of just ignoring that idle threat (who would believe him over a cop???) he sides with the killer and frames Tanjas boyfriend
So...there MUST be a logical explanation for this...and there is!
We find out that the killer had sex with the underage girl.
Its also implied that the killer has some erectile dysunction or hang up
We see the cop take Tanjas friend for a ride in his car and grope her
We see the cop try to make moves on the receptionist when she shows him her kitten (LOL very Freudian...a little pussy)
Its entirely possible that the cop sides with the killer because he realises they are alike sexually and just as he has made a mistake killing his partner understands that the killer made a mistake killing Tanja