In my view, this is utter nonsense. Indeed, it is not merely utter nonsense, it is literally nonsense, and it is not something that I think any intelligent person can reasonably endorse.
I find myself generally in agreement with him, and I'll admit that I'm surprised that a some fairly well informed Catholics seem to consider the idea of such crimes being the result of possession to be worth considering.
While I certainly have rationalistic tendencies, my skepticism here has nothing to do with a denial that demons in general and Satan in particular do exist and can perform actions which directly affect people. (Perhaps I'm overly influenced by folk literature, but I also don't necessarily have a problem with the idea of there being more ambiguous angelic-in-origin creatures banished to the world for refusing to pick sides.)
However, I do have serious moral issues with the idea of someone committing some crime because he is "possessed". Unless the term is used to suggest some sort of persuasive power or influence (which almost seems to rob the term of any meaning -- making it equivalent to "tempted"), it seems to me that the term "possessed" refers to a situation in which some other being than the person who owns a given body is directly responsible through the exercise of his will for some action or set of actions that that body takes. Thus, to say that the gunman was possessed when he committed his crimes would suggest that it was not the gunman's will which chose to pull the trigger again and again but rather the demon's.
Now only does this seem disturbingly problematic from a moral point of view, it also does not seem in keeping with the instances of people being "possessed by a demon" in the scriptures. In those instances, the possessing demon does strange or disturbing things (such as crying out, exhibiting unnatural strength, engaging in wild behavior, etc.) which seem designed to cause fear and despair in the person possessed and those around him, but does not go around simply committing crimes using the possession victim's body.
To the extent that one chooses to see demonic action at work among humans, I think it must be seen (if one is to remain reasonable) in terms of tempting, warping perception, and instilling fear. All of these have the capacity to corrupt the will and turn it from God's will, while simply taking control of a body to commit crimes achives no such thing.
Also, I see something of a moral danger in the tendency of people to think "No person could ever willingly do such a thing" in regards to some notable crime. Assuming that no one would do such a terrible thing makes it all to easy to assure oneself of the inverse, "Anything someone willingly does cannot really be so very bad."