Death Coil is Better Than Polymorph for the Following Reasons:

Death Coil does not need to be researched while Polymorph requires 2000 Gold to research.

Death Coil only requires 100 mana while Polymorph is 200. Death Knights are more likely to have enough mana to cast Death Coil than Mages having 200 mana to cast Polymorph.

You can cast Death Coil soon after Death Knights are trained from Temple of the Damned while you must wait a long time for a Mage to build up 200 mana. When time is an issue such as when you're being attacked by Dragons, this time difference can be deadly in the case of the Humans.

Using a Death Coil takes the hit points of another unit and adds the hit points of the Death Knight. You could use this to heal your Death Knights which is an added bonus.

You can use two Death Knights at 100 mana to kill one Gryphon while two Mages with 100 mana would not be able to cast Polymorph yet.

Polymorph only takes one hit to kill an enemy unit while Death Coil requires two casts to kill an enemy. Sometimes a Death Knight or Mage is being charged by something and they only have a short time to cast a spell before they are destroyed by an enemy unit such as a Knight. In this case, you can often cast Polymorph to kill an attacker but if you tried Death Coil, it would take to long and would be too late.

Sometimes a Death Coil can have the end result of no damage to the enemy. A big problem when using Death Coil is you only are able to use one cast before the enemy runs away and Heals.

Bloodlust Advantages

Bloodlust is an offensive weapon while Healing is mostly defensive.

Bloodlust can be cast on units before invading. Healing is only useful sometimes during or usually after a battle only after hit points are lost.

Bloodlust makes your Ogres 3 times more powerful than Knights. Most likely you can kill the Knights before they can heal themselves.

Bloodlust is scarier sounding than Healing.

Healing allows you to use hit and run strategies with no damage. You can attack with Knights, Gryphons, Mages do some damage, then retreat and Heal. The enemy is damaged but after Healing, you have no damage. If you can keep this up without the enemy doing anything to stop it, you can wear down a superior force until you can overcome it with your own troops. Healing saves money in replacement costs. However, Healing doesn't come into play except in the case of survivors in battle, Mages and particularly Gryphons.

Healing can sometimes allow a large amount of hits on one guy without him dying. As long as there is mana, you can keep Healing. You can set up a choke point where Paladins heal the units at the front of the choke point so they can take on large numbers of attacking enemy units.

Healing allows useless troops to become useful again.

Slow Advantages

Slow cuts the number of hits the enemy can do in half unlike Haste which doubles the number of hits only for Dragons.

Slow cuts the walking speed of all units in half which allows Mages to run away.

Slow is a more offensive spell than Haste.

Slow is very annoying.

Haste speeds up the casting of Death and Decay.

Haste allows Death Knights to move as fast as Flying Machines.

Haste speeds up repairing, chopping and mining.

Haste speeds up Dragons to about 23 and doubles the number of their attacks.