I think that you mean Demon Killer. Monster Slayer, is an enhanced Power Attack vs. Large+ critters.
I personally read it as being more specific. With the feat, if you have defeated a ghoul in single combat, then you don't ever have to make a Terror save vs. another ghoul. Without the feat, you always have to make the save (although at +2) to act unimpeded.
And I will change the last paragraph on p313 to read "Monsters in this section all force Terror saves. At the Games Master's discretion. . ."
Well, it also seemed to me, that if you were walking along and saw a ghoul, and subsequently fainted in fear (however, your compatriots were able to fend it off), that maybe you shouldn't be immune to the Terror of seeing another ghoul later even though you have encountered one previously.