I've run Descent 2nd edition as the Overlord and find it utterly impossible to win! Killing the heroes seems like a useless ventrue. All killing a player does is cost him/her a turn to get back up or even less if they get help or healing from another player character. The 1 card reward for the overlord is a pretty pathetic reward for killing a player character. The game also does not scale correctly with a lot of players. A 2 player party, the overlord gets 1 card a turn. A 4 player party, the overlord gets… you guessed it 1 card a turn! 4 player games, the Overlord has no additional spawns so while the Overlord starts with more monsters in a 4 player game, his ability to replenish his minions does not change at all. The Heroes also majorly outclass any monsters you throw at them and often it takes focus firing to take down even one hero. I've never dropped more than 2 heroes in a single turn! My other gripe with 2nd ed is that they got rid of Conquest Tokens. The overlord should be able to win through killing the heroes, without that victory condition, the game becomes a rush for objectives where the Player Characters have major advantages to achieve. I would have perferred that 2nd ed Descent kept the game balance of 1st ed descent and allows the Overlord more power than the PC's and thus allowing the Overlord to provide an adequate challenge. Every time I've run the game the heroes have walked all over me. I have a feeling the slant of the difficulty is toward the PC's and not the Overlord. Then again I haven't yet got a game to chapter II in a run, where the monsters get a stat boust, but even then I don't think the difference will be that grand as the PC's will be more powerful at that stage as well. In the end, the Overlord is heavely gimped in 2nd edition.
Killing the heroes is not your goal. Killing the heroes is not always a good idea.
If 2 heroes are adjacent and you only kill 1 of them that was already fully exhausted (fatigue = stamina), then as you said you get just 1 card. Hero adjacent to killed wastes 1 action, but hero heals 2-6 damage and 0-2 fatigue. Hardly a wasted action. If heroes are not adjacent and you kill the one in such spot that the rest of party either can't or will really not want to go to in order to rez, killed hero was fully rested, then you still gain 1 card, but also give that hero fatigue and not only disable him for his next turn but also disable his abilities for turn after that and what stands in the way of using that next turn to kill that hero again? Especially if he gets no luck during stand-up and rolls 2. Killing all heroes at once gives you one full turn for free, and if you managed to do that it is very likely you will do that again.
The game is about speed and race, not about killing. What is the difference between hero standing up for 1 turn, or hero attacking for 1 turn, killing your monster, when you get it to spawn at the begining of your next turn? A little one. Maybe let's look at the "Fat Goblin" encounter 1. In the begining you have 1 open group of monsters. Heroes can: (a) kill monsters (They will likely succeed.) or (b) run. If they kill your monsters, your goblins are 1 turn ahead in stealing crops. If they run, you can always stop some of them with tripwire/pit trap. And, you get a monster group with some attacks that can and will damage heroes and that will hurt them in encounter 2.
Scaling: Is ok. True, usually your reinforcement do not scale, but if you don't let hero wiping an entire group, then this is not an issue. Sure, sometimes you get no choice, but that requires all heroes effort and wastes their time, which in turn might win you an encounter even if losing monsters. Also with some tougher monsters -> you still get 1! If you take zombies -> you get 1 zombie per turn which against 4 heroes is not great. But if you take dragons, you get 1 red dragon per turn which is great even agaist 4 heroes, especially if they went melee heavy Also some quests do not have any reinforcements, so there is no scaling here really. Anyway, the game is not so simple and scales on many different layers and some of these layers are not obvious. While it is a little bit different game with 2 and 4 heroes, it is still balanced IMO.
The 1 card per turn is auto-scaled. This is due to 2 facts: 1) you have more attacks and you deal more damage, while individual heroes still have the same amount of hp. There are more heroes so you are theoretically able to kill more heroes per turn. -> this leads to more additional cards on average. 2) cards themselves scale. For example Dash in 2nd encounter of fat goblin is great. No matter how many heroes there are it can win you the encounter. Dark Fortune on failed test with Splig is a big chance to save that wasted turn and speed up the encounter in which time is not your ally. Dark Might on your red dragon in order to dish out AoE damage and hit each hero? Much better if there are 4 players. Especially if you get to kill some of them at once! Critical Blow or Frenzy will still kill 1 hero and replace themselves with additional card. Some cards are mutli-target, some cards work better if you have stronger or more monsters. Some will separate heroes for easy repetetive kills from your monsters, some will punish heroes for sticking together. There was a case, where a card scaled "too good". In case of Unholy Ritual it allowed you to draw 1 card per hero, which was so overpowered, that they changed it to constant 2.
Hero stronger than monster:
That is generally true in act1. This is why you have more monsters than there are heroes, extra cards for free and reinforcements. Some things to note:
Hero start with 1 gray defense die. Some monsters have 1 Black, and some 2 Gray. Most monsters also have 1 gray die. Monster's abilities are for free and do not cost fatigue Leutenants are stronger and in their strongest form (4 heroes) could beat hero 1vs1. Some heroes are just not good against certain type of monsters (goblins can kite to death dwarf zerker, dragon vs melee blue+red) Some monsters deal more damage than heroes except their one attack. Buffed with cards monsters can easily outdamage hero. For example stupid red goblin deals more damage per attack than Jain Fairwood with Yaw Shortbow. Monster hp is very low. (Although, some heroes don't shine here either -> namely Leoric)
You need to pay attention and not let the heroes finish off entire groups of monsters. Your goblins should be afraid to come closer to heroes, and should shoot from range. Kite melee and take down ranged first. Don't make it easy for heroes to kill your creatures, force them to use fatigue or move. Maybe it is good idea to sometimes run away, and join forces with stronger monster groups? Goblins shooting, retreating and shadow dragon blocking entire corridor? How many such turns the hero party will survive? Master zombie can come in, immobilize and go away. This is better than killing off melee each turn. Unless some other hero helps the poor melee character, the zombie will keep him in one spot doing nothing.
Well, 2nd edition of descent is different. I think it adds a lot of depth and replayability and is not a simple hack&slash game. Combat is an important part of the larger game. Overlord has an evil scheme and is trying to accomplish something different than just killing some band of random adventurers. I agree with Dam. Give yourself some time to learn and you will see that it is the heroes will have a very hard times winning any campaign.