Will addiction to watching pornographic films and clips, despite praying, reading Qur'an, and doing good deeds, send its perpetrator to Hell? And does seeking forgiveness after a sin erase misdeeds, or does planning the sin with the intention of seeking forgiveness change the ruling?

All sins – except for polytheism (shirk (definition)) – are subject to the will of Allah; He may forgive them if He wills, or He may punish for them if He wills. Seeking forgiveness (istighfar) is legislated in Islam and is a cause for forgiveness, especially when accompanied by remorse. However, seeking forgiveness merely with the tongue while persisting in the sin is not sufficient. One must repent from the sin, not take it lightly, nor return to it, and fear an evil ending.