Your definition actually bugs me.
"Good" and "bad" are values that add another layer to the discussion. I'd appreciate "max. fun" or "max. competition" instead, as they do not evaluate the Keepers actions instantly. One CAN play to win, killing off investigators left and right without being annoying or "bad". I know you mean the evil kind of bad, but I don't think "nice" and "evil" have a lot to do with it. "Nice" Keepers can still kick you in the groin and spawn a Maniac on you if you're down, as long as it fits the theme.
I, myself, try to make it the most fun for everyone, including myself. And since it's Cthulhu, "fun" basically includes a) risk of death, b) permanent danger and c) horrible, horrible things everywhere.
Usually I'll try to keep the game going until the finale. You know, the last Event card imminent and stuff. If "Kill two investigators" is the objective and I COULD kill them off within 5 turns, I DON'T. Because I don't want to gather people around a table only to seriously annoy them for 20 minutes. Got better things to do, really.
MoM is no RPG, of course. But it's not a purely competitive game as well. If you want competition, might I suggest, I don't know, Chaos in the Old World. Everyone's equal, everyone wants to win, NOT screwing someone over doesn't improve the experience.
MoM is a game centered on narrative. You tell a story. Like every story, even MoM games can include suspense, if done right. Maybe the story allows you to permanently attack the investigators. But if it is not 100% necessary, why would you? Hobo moves, gets attacked by Zombie, evades, gets attacked again, evades,... BORING!
Try to throw different encounters at them.
Maybe do nothing except picking up cards for 2, 3 turns!
"Presence" is a great card to create suspense. The players realize your increasing your Threat only to unleash some horrible monster, see the Trauma cards increase in your hand... add in an evil laughter now and then, a "just as planned" when they decide to enter the Basement and there you go, your players might be more afraid than if you'd spawn a Zombie each turn.
Just think about how you'd want the Story presented as a movie. A classical Horror movie, not the new kind of "People enter house, get knocked out, wake up and the next 60 minutes are filled with their bodies being chopped to pieces". People don't enter the ghost house and get attacked instantly. You don't see Zombies in the first couple of minutes in Resident Evil movies (unless it's a flash back).
Let them explore the entrance hall, the next room, maybe add some Mythos card to play. Spawn a single small monster, maybe the turn before the 1st Event is resolved. Build up pressure, maybe slow down after turn 10 to increase pressure again during the finale. Don't crust their hope immediately. It's more fun to crush it if they really think they're gonna win, after all
In my opinion, the "Keeper can win" part is there for two reasons:
- One even my players enjoy: The Keeper does not necessarily win if the investigators lose. This has some impact on how the Keeper plays the game. He cannot just slow investigators down, keeping THEM from winning. He needs to manage resources as well. Moving monsters somewhere through the Mansion just to trigger an event might not sound hard, but when you need to slow the players down AND do something on your own, you're in a position of conflicting goals. Those are awesome! Dilemma! Greatest way to enforce players (in this case the Keeper) to think about the game, try new stuff, etc. If the Keeper'd win just by stopping the investigators, some stories would come down to "move every monster to room X, wait for the investigators to find the Clue that sends them there, party party". Sure, a couple of Stories STILL end up that way, but there is muuuch more stuff to do in most others.
- Offering the Keeper a way to win stops the... let's call em "really nice" Keepers from doing nothing. If the game would read "Keepers should try to make the game fun for their players", some bad (as in: less skilled) Keepers might interpret that as "create a super happy fun Mansion where everyone gets a Shotgun in turn 1 to dance with". Bit sarcastic there, but that's basically what I gathered in several other threads when "Keepers should stomp the players ASAP" guys talk about us Storytellers. If the newbie-Keeper sees a card that tells him to do X, he might try to do X instead of creating super happy fun Mansions. Way to go.
HAH, just found the graph I was looking for. It's based on a talk from John Romero (the Game Designer).
Of course this graph relates to a whole video game, so you won't manage to get so many breaks in a single game of MoM. But permanently increasing pressure -> tiresome. Allow them to ease up a bit in between fights. You'll need the threat anyway