The argument that some evil is necessary is used to defend some evil acts. For example, someone might say that because there are evil people in the world trying to do evil, the good must use evil, when there is no other way, to restrain the evil.
There are two problems with this idea (but the first is maybe just an explanation of the second).
Firstly, evil has a tendency to escape and overflow such situations (eg. The abuses by militaries of their enemies and innocents). This means that it is doubtable if evil policies ever decrease the total evil. ‘Fighting fire with fire’ doesn’t work literally (ask a firefighter), and I don’t see why it would metaphorically. Is there anything in existence that lessens itself? Anything that drives itself out? Anything that defeats itself? Why should evil be unique in this way?
Secondly, I have never heard of anything in existence, where its own existence is an argument for its necessity. Please tell me if you can think of one. Unless evil is a unique substance in this way, or I’ve overlooked a similar substance, or it does not truly exist, it cannot be necessary because of itself.
Evil can’t defeat itself, nor excuse itself.
The three options I see are that
1. Evil doesn’t exist
2. Evil isn’t evil to evil people
3. Evil is never necessary for the sake of evil.
Religion (and morality) refuse option 1. Evil exists as much as good does.
Jesus rejects option 2., when he tell us,
‘but I—I say to you, not to resist the evil’
‘but I—I say to you, Love your enemies, bless those cursing you, do good to those hating you, and pray for those accusing you falsely, and persecuting you, that ye may be sons of your Father in the heavens’
Jesus and Paul accept option 3. when Jesus tells us to
be perfect, as your Father who is in the heavens is perfect.
and when Paul clarifies the Christian position to
Be not overcome by the evil, but overcome, in the good, the evil.
God bless you