Experiments on Guardian Ascended?! Thats EVIL!!!!:
Those experiments were performed on the failed Guardian Ascended that could not defeat Regulos. So what use are they now? The Defiant finally figured out how to create their own Ascended and are taking matters into their own hands. They know that the Guardians will fail, that is why they must go to the past to intercede and stop Telara from impending doom.

Do the Defiants believe in the Gods?:
Yes, they believe that the Gods are real...do they have trust in the Gods to stop Regulos...no, as you can tell from the Defiant-side tutorial gameplay.

Defiants = Heretics? :
Because they use magitech to live their lives? Because they don't believe the Vigil can save Telara? They are called Heretics by the Guardians because they are relying on themselves for salvation not the Guardians' Gods. The Defiants have paved their own road and do not need Gods to save them.

So, who are the good guys? bad guys? :
There is no good or bad in this game. There are two polar opposites. Guardians put faith in the Vigil. The Defiant put faith in each other and stray away from Gods. They are both trying to save Telara from Regulos, only that the Defiant know what the outcome is if they don't stop him first.