How successful is a sting
An under cover LEO does not blow their gig to make an arrest or enforce the law unless someone's immediate safety and more so their life is in danger. If the LEO has been able to pass as a college student, I'm willing to bet that there are much bigger fish to catch on campus than a few couch killers. Such as drug dealers, thieves, rapists, etc.
You (and many others in this world) are making a lot of assumptions when you've not ever walked in the shoes of an LEO, are familiar with the laws, or know the department's SOG. It is little different than being an armchair general to soldiers in the field. Hold fast your tongue and remember that you simply do not know except what a few pictures show. 
How successful is a undercover operation if they are only going to be arrested for Destruction of Property or arson in the end, after the property is destroyed?
I agree with you that there is much bigger fish to fry on campus with undercover officers, but In the case of these "couch killers", perhaps the presence of officers or campus security on the street before the craziness began would of prevented the destruction of property.
