Before drop 3 bloodthirst was terrible in PvP. Well lets be honest, it wasn't good at all really. If you had the DPS you could force a stalker to run it in raids.
With drop 3 we have seen bloodthirst get a overhaul and boy does it have some fancy tricks! Currently Base tier does the job nicely and the damage at tier 4 brakes stealth so your opener can get hurt by this. You can tier it up to T3 for some extra lifesteal but with lifesteal getting effected by the healing debuff its not worth it.
What bloodthirst does allow you to do is get a 108% damage boost for 8 seconds.
What to rune for?
Even at basic PvP blues you can still deal massive amounts of damage. Your best runes are going to be PvP power. Rune sets are good but to really take full effect of this build you need 100% PvP power or more. You should prioritize the following:
T8 is useful because it has shield overload. For 6 seconds! Im sorry but that is crazy good!
You are able to run with out ruin and T4 pounce, and add T4. It depends on your play style.
AMP variation are going to be based on how you want to play.
Originally I had this for my AMPs but Devastate is crazy good when you have a target at low HP. You don't need extra armor pen because it completely ignores armor :D
Bloodthirst
Ruin
Spam Impale Punish when your at the correct Suit Power
Stagger
Stealth
Impale
Out of Stealth or Stealth on CD
Bloodthirst
Spam Impale Punish when your at the correct Suit Power
You want to wait for stealth to get back up before you use Ruin, once its back up stealth and wait for cooldowns.