Decent question. Keep in mind that to make a kill you have to take the chance to get killed. The ability to stomach that realization and still be an effective killer is what makes an ace.
Plenty of guys (we're talking in the past here, pre-1960) went out, flew their tours and never blew the tape off their guns. Does that make them ineffective- not ace worthy, no, definitely not. They may not've had the opportunity (flying wing, ground attack or in an area where air to air was over- Joe Foss's second tour is a good example). They also may not've been the hard core killer type. And high scoring aces either are or become that type.
But back to the part where you have to take a chance to get killed to get a kill. Some people can't get past that point. Doesn't make them a coward or any of that, those types are generally weeded out. But guys who make kills tend to be pretty cold blooded. Mannock or McCudden once said that they were hired assassins, nothing more. It would be worth keeping that point in mind.
One last point, the guys who get whacked down are generally the newest and least experienced. The US Air Force did a study and found out that a pilot had something like a 25% chance of being shot down on any of his first five missions (note- odds posted could be as high as 50% but I've had a few beers and the first thing to go is "Mr Memory" so take that part with a grain of salt- Note 2. Odds per mission are not cumlative, 25% per mission on each of the first 5 missions) and thereafter the odds of survival increased (or of getting the chop decreased) to the point of a steady 5% per mission (average on that last one, flying an escort mission to Berlin is going to be a more hazerdous, in general, than one to the Normandy). A good ace/killer is going to quickly figure out how to spot the neophytes and go after them. The experts or hard core are much harder to get. Replacements die in groups, veterans die alone.