They are pretty damn bad.
Sacrifical pact : Obviously terrible if used on a friendly minion, great if used to kill an enemy minion, but considering the only thing that runs demons is Zoo and thats 6 demons only, useless versus Handlock which might cause bad mulligan.
Corruption:Takes a whole turn, allows the enemy to trade the doomed minion or at least hit you in the face with it.
Mortal coil: A poormans version of wrath and shiv.I dont even run it in my zoo.
Demonfire:Might be good IF there were enough reliable demons, the utility is nice so it can be argued if this card is good or not.
Drain life: Laughably bad, 3 mana for 2 damage, the healing is irrelevant you need the card for removal not to do bad 2 for 1 trades(and mana costy too).
Sense demons: Could be good if demons were relevant, but that is not the case(and probably wont be after Nax too).
Shadow bolt: Arguably decent, for one more mana you have a fireball that does 6 damage but can also go to face, or swipe that will also do 1 more damage and can go to the face.The 4 damage instead of 5 is a huge drawback and it would make sense for it to be 5 considering its limited to minions only.
Hellfire:An aoe that hurts your own board and hero.You want to make a spell powered combo? Too bad your minion dies(even azure drakes).You want to clear the enemy board and go for face? Too bad, your minions might be dead too.
Siphon Soul:Well how i dare one of the best cards in handlock deck and i call it bad? Well its in the handlock deck because there isnt anything better.
Something simple as a Polymorph + Hero power is the same mana cost and card cost but on top of that it silences the minion it kills, and still polymorph isnt played that much.Assasinate costs 1 less(yes it doesnt heal) but its still rarely/never used on higher levels.
Bane of Doom: 2 damage for 5 mana is laugable, the secondary effect triggers only on a kill and its still a random, and on top of that Pit Lord and Doomguard are NOT on the summoning list.
Twisting nether : A complete board wipe is great but costing 8 mana is quit a lot considering the fact it also destroys your own board, this means that you are giving the initative to the enemy during late game turns which can be devistating.
Warlock is probably the only class that has so many bad spells how can you run anything that is not Zoo or Handlock? If its not for the good hero power(that makes those decks viable) the warlock class will be BY FAR the worst in the game.