Is the sentence "But he was still on the watch" in the short story "The Wrong Category" ambiguous? Can the reader infer that Barry is on the watch for victims because the sentence that follows is "Next to viewing the spots where the six had died, he best enjoyed singling out the next victim"?

Mystery author Ruth Rendell is a masterful writer of plot twists.  She knows exactly how to make everything seem obvious, right up until the very last moment.  In her short story "The Wrong Category," it is evident that she has singled Barry out as the most probable culprit of the recent murders that have been committed in town. Every piece of evidence seems to support him as the killer; he walks around at night, he visits the crime scenes, he watches women which fit the killer's "type," and he is lonely, awkward, ugly, and weird. Barry has all the makings of a murderer.
Because Rendell is so masterful, however, she leaves just the slightest bit of doubt that Barry is in fact the killer.  That is why this line from the story is so ambiguous. "What if he is not the killer?" we ask ourselves.  What if he just likes playing weird games because he has nothing better to do?  However, most of us do not go around at night to spots where murders have been committed to try to pick up girls, especially if we are not trying to get killed.
Why would Barry, a normally awkward guy, choose to get his thrills by imagining who the killer's next victim might be? We have to remember that Barry is already a strange guy, so it is not entirely implausible that he would do so. However, interfering with a murderer's case when you are not the murderer puts you directly on the actual murderer's radar and could also get you killed. I would say we can certainly use these lines to surmise that Barry might be a killer who is on the hunt for his next victim.  However, as I pointed out before, Rendell leaves a bit of doubt. She leaves just enough to confuse the reader and cause him or her to question what seems obvious.