Some people don't want to lower the difficulty slider. I've touched my difficulty slider once in the time I've played Oblivion (over 4 years now, I bought it the day it came out). And that was to raise it higher than the default on my second time playing. To me, lowering the difficulty slider feels like giving up.
In this case, I tend to agree with you, because the gatekeeper is supposed to be tough. In fact, I'd say the gatekeeper is supposed to be very, very difficult to kill without the help of the characters mentioned above.
Generally speaking though, I view the difficulty slider as being an "adjust for believable combat" slider. If you believe that a common bandit should be more difficult to kill than your character, raise the slider. On the other hand, if you believe your character to be strong and durable, you might want to lower it a bit. Go very far from the middle in either direction, and things can get silly. (Some people believe in silly.
)
The problem with moving that slider is that it doesn't really change the "difficulty" of the game at all. It just adjusts the relative damage dealt by you versus your enemies. It doesn't affect other NPCs, either, so if you push the slider way to the right, you can find yourself in the ridiculous situation where your heavily armored and weaponed character can't take out a fur-clad bandit, but some random NPC on the road can do it with their fists. :huh: