Does slacking off from performing the night prayer with a hundred verses after having performed it, and limiting oneself to three rak'ahs, mean that God is displeased with him, or that He has come to dislike that act of worship from him?

This slackness in acts of obedience after becoming accustomed to them is not an indication of Allah's displeasure; rather, it is blameworthy if it is without excuse. One should persist in prayer and night vigil, for the most beloved deeds to Allah are those that are most constant, even if they are few, and discontinuing acts of obedience that one has accustomed oneself to is disliked.