Why would God allow Satan to do stuff?
There was a debate on Nightline a while ago (YouTube link) which discussed the existence of the Biblical character Satan. One of the persons in favour of an actual Satan said that she became a Christian because such a character existed, or in a skeptical explanation because she experienced a negative force that acted like Satan (i.e. self-destructive urges in human nature).
I find this statement interesting because it seems that many, many people become Christians because there is a sort of self-destructive or selfish force within people. What sometimes happens is that people's lives are falling apart and they need to make a choice between giving in to the self-destructiveness, and falling into an 'abyss', or giving their life to the Jesus of the gospels. In this way many people decide to do a complete turnaround and commit their lives to Jesus.
I see hence a reason why God might allow humanity to be tempted beyond what would 'normally' occur. A Satan character could magnify and make more extreme the self-destructive impulses within us. A lot of the time this would just be bad. But sometimes, through being forced to make a stark choice between life/death, becoming like Jesus/self-destruction, 'Satan' would actually accomplish the opposite of what is intended (something like, 'Hey, this is what will turn my life around and give my life meaning and purpose').
This seems like a reason for God to allow a 'Satan' to tempt humanity, if that's what it would accomplish. But I think two conditions need to be satisfied from a secular moral point of view:
1. The first condition would have to be that people are being saved from an ultimate separation from God in the process, which is, in reality, a really horrible fate.
2. The second is that this process is the ONLY thing that will cause some people to change their mind and choose to live forever with God rather than apart from God.
Presumably, if there was another way to save someone, then God should employ that way instead.
Looking at the second condition, what's a list of things that often make people want to be with God rather than be apart from Him? Often people are drawn to Christianity by feeling some sort of emptiness, lack of fulfillment, a feeling of hopelessness or pointlessness to life without God, discontent with the whole idea of serving oneself rather than God after seeing the 'bad side' of sin, the idea of perfect relationships in the afterlife rather than messed up ones now, a hope for 'another kind of happiness' that is eternally fulfilling and great, as well as other reasons.
These thoughts are negative thoughts that Satan could easily magnify (i.e. in Satan's desire to make life harder for people). They are also thoughts that tend to encourage a lot of people to follow Jesus and be with God forever (note: one reason why I associate 'follow Jesus' and 'be with God forever' is that it makes sense to me that you could only ever go to heaven as a free gift and I don't know any religion that teaches this apart from Christianity).
So if Satan can magnify these thoughts, and having these thoughts sometimes makes the difference between someone choosing to be with God forever rather than be apart from Him, then does Satan have a useful role? Can God use Satan? It seems so.
So allowing a 'Satan' to do stuff under these two conditions and only if these two conditions are satisfied could make allowing Satan to do stuff an overall good thing, and these conditions make some sense.