Ehrm.. No. Guards kill stuff by surviving. A frenzied DT does kill very fast even without Void of Madness...
Covenant of Arms is the best Covenant, because its damage is activated by your offensive hits.
Covenant of Pestilence's damage is activated by you getting hit. It does add a bit armor but the quicker you kill your targets, the less damage can they do to you. You will do less damage with this covenant active over Covenant of Arms.
Covenant of Invulnerability is only ever needed in protection fights where you need more protection than 40%. You never ever need this in a leveling process.
You do not have to take any points in Depravity at all until you get to level 60, and then you can feat the aoe stun (Mind Shatter) if you need it. Feating Soul Barrier is nice indeed but you will be out of mana in no time if you also use Pact of Malacodor (which I highly recommend, and is one of the main damage sources).
Levels 10-25:
http://goo.gl/3P38lN
Levels 25-40:
http://goo.gl/bStn3C
Levels 40-60:
http://goo.gl/GfLmz5
Levels 60-80:
http://goo.gl/p1Pm0T
Those are leveling builds with good dps. If you need to tank in a group instance, use the multi specialisation you get from a trainer. Black Castle is the first sixman instance you can do, but more likely, Cradle of Decay, Sanctum of Burning Souls and Toirdelbachs Tomb are where you will get your first tanking experiences. Those are at level (+/- 1-5 levels) 38, 40 and 48.
Use this as a default template for tanking. You get the idea, grab armor, protection and improved defensive stance from General tree. Spend to your liking in the others, but always have Pact of Malacodor and Covenant of Arms.
Level 40+:
http://goo.gl/bGJz40
Level 50+:
http://goo.gl/ZkmmZ4
Level 60+:
http://goo.gl/5uWErC
Regarding combos, try to keep a rotation where you always have tap buffs from Leech Life and A Soul for our Feasts active, and the mob debuffed with Unhallowed Blight. Blood for Aid is the "bread and butter", use that when other things are on cooldown or you wait for the right time to put up buffs again.
Remember to buff up, you should at all times have active Sadistic Soul, Pact of Malacodor, Drain Life (/Essence if you got Soul Barrier up), Aura of Dread Fury (Infusion if another DT in your group has it feated in Desecration) and Covenant of Arms.