But do Christians *really* need to discern evil? (Surely a Christian ought to give people the benefit of the doubt?)

Do Christians really need to discern evil? Yes! 

Christians really do need to discern evil, in the sense that evil is those who have taken the side against God; and Christians need to know who is on that side. 

When somebody (or some social institution) is on the side against God, we need to take a different attitude to them - towards what they tell us and what they do - than if they were on the side of God. 

Since we all live in society, and all must deal-with other people and with social institutions - we simply must discern which of these people and intuitions are overall and by motivation working against God, the Good and divine creation. 

And if we get it wrong, then we will will have joined the side of evil. 

If Christians mistakenly 'give the benefit of the doubt' to people and institutions (instead of trying to discern the truth as accurately as they can) - they will falsely regard actually-evil people and institutions as Good. 

And then these (self-identified) Christians will believe and propagate Big Lies; will act dishonestly in support of evil; and consent to be manipulated strategically, in multiple and coordinated ways, to destroy the virtue, beauty and truth of divine creation... 

Which is, of course, normal and mainstream behaviour nowadays among the millions of people who suppose themselves Christian but who actively serve Satan. 

That's how important it is to discern evil.