Ok - I'll try to go a litle bit further into "Warlock is a good combo".
But first - if you wish to compare Lightning and Fire effect, you should compare Daze to Burn and Stun to 2 Burn (more or less) - and Daze and Stun are more deffensive abilities, so Fire has bigger pure damage.
Also - many of nature's spells are weak to fire, so even if Beastmaster may have better pet, he has big weaknes vs. Warlock.
Pentagram - it gains mana whenever an enemy creature is damaged by friendly creature - so it gains mana when you do, what you'll be doing anyway - attacking your enemy. Giving mana to Temple of Asyra is trade-off (your Clerics are not guarding/attacking/healing). For Wizard, on the other hand, creatures are not so important, so spawnpoint is less worthy investition.
Gate to Hell - meele +1 for demons isn't much for 12 mana, but giantic attack on almost whole arena is something worthy consideration. Mayby not for 24 mana, but hey! it gives your demons meele +1 and gives you some tactical adventage with possibility to summon creatures in the other place.
Pentagram and Blood Reaper - those gives you benefits for doing, what you want to do - killing your enemies.
Pentagram is described above.
Blood Reaper costs you no mana, heals you when attacking (so when is doing it's job) and since your opponent would be damaged anyway, Bloodthirsty is almost pure bonus. Sure, you will not want to make Blood Reapers again and again, but Dark Pact Slayer and Helion both has staying power (especially when your enemies are cursed). And you have more life than anybody else, so you can pay it one or twice.
Goran has Bloodthirsty too and you have another strong offence.
Demonhide Armor, Lash of Hellfire, Battle Skill, Fire specialisation - those ake killing your opponents fast and easy.
Demonhide Armor isn't the best deffensive equipment, but it adds a little to your offence.
Lash of Hellfire is simply strong - esspecially with your Battle Skill.
Fire school adventages are described above.
And again - your life makes it easier to go fully offensive.
Moloch's Torment, Curseweaving and Ring of Curses are another combo and not really tradeof, since weakening your enemy is something you like to do anyway + they do not cost much. Curses (and more life) are Warlock's substitute for much weaker healing skills - weak opponent would not damage you much.
Plus you have your Blood Reaper.
There are some good dark magic for healing too:
Drain Life - you are healing and damaging in one action. No trade-offs.
Same with Death Link - you are closer to win and further away from lose.
Helm of Fear - this is stronger than you think. Why? Because it's ability isn't Deffence -> you can have it and deffence. It is always good to have two chances to avoid attack.