The Feywild and Shadowfell are planes parallel to the Prime material plane. Though they are usually depicted as good and evil planes respectively, is there evil that resides in the Feywild, or good that resides in the Shadowfell?
Though the Feywild might generally be depicted as a wondrous place full of life and vibrant colours while the Shadowfell a dark, dismal place of shadow and death, this hardly means they are (meant to be) inherently good and evil.
The TL;DR version is simply that evil certainly resides in the Feywild and good certainly resides in the Shadowfell.
The Feywild is home to many malevolent and typically evil beings. Hags, goblins, orges, giants, werewolves, blights... You name it. The 4e Heroes of the Feywild (HotF) suppliment even states:
These villains can be as fundamentally evil as devils or demon lords, and they wield powers of comparable might.
While there may not be a lot of information about the Feywild in 5e there is:
Seelie and Unseelie Fey.
[...] Seelie and unseelie do not directly correlate with good and evil, though many mortals make that equation. Many seelie fey are good, and many unseelie are evil...
There's plenty of evidence that can be found in the 4e books Manual of the Planes and HotF and the 5e DMG to suggest that the Feywild is certainly not full of only good creatures.
There seems to be less direct evidence that there's good in the Shadowfell but one quote in 4e's The Shadowfell - Gloomwrought and Beyond, I think, strongly suggests that even the Shadowfell is not entirely full of evil beings.
The Shadowfell is not inherently evil, but evil creatures are strong and plentiful there.
Evil may be abundant but it is never stated that only evil beings ever reside in the Shadowfell.
The Manual of the Planes also states:
For all the Shadowfell’s dismal reputation, it does have bright spots where those who fight against the malaise and the chill live almost ordinary lives. In fact, for many visitors to the Shadowfell, the plane has more in common with the natural world than it has differences.
Much like the natural world there is both good and evil.
I don't know about the Shadowfell, but yes, there is evil in the Feywild.
Page 10 of Heroes of the Feywild, referring to the Feydark (the Feywild's counterpart to the Underdark), states, "Food and water are plentiful for the underground inhabitants, but so are predators and malicious fey that inhabit the subterranean realm."
Also, page 12 of the same book states, "Though they have no true representative among the Court of Stars, there are fey creatures of evil, maliciousness, and corruption that are collectively known as the Unseelie fey. Many of these creatures draw power or are shaped by the power of shadow, drawn from the Feywild’s opposite counterpart, the Shadowfell."
Short answer? There is evil in the Feywild and good in Shadowfell
In fact, most intelligent races in the Shadowfell, including Shadar-Kai and Dark One's, appear to be capable of good or evil. Shadar-Kai were, in 3e, usually evil, but in 4e they are unaligned. Cold, pitiless, but not evil. Dark Ones are thieves, not murderers, and chaotic neutral.
The Feywild is easy, and in more recent years the game has taken to the older folk tales for inspiration, not Alice in Wonderland or Disney. Hags and such are flat out evil, and other monsters fall between Chaotic Neutral, Evil, and only occasionally good. In fact, most 5e stuff has evil or neutral fey creatures, at current; the majority of creatures are, in fact, Hags.
The Sidhe themselves are more "Human-Like" intelligence's, the princes and queens of the Feywild, and can be thought of (and have been called) medieval versions of aliens. Different, but somewhat comparable, intelligence. Often sinister, sometimes benign, sometimes benevolent, but different.
Until more is sent to 5e we can't be sure, but its almost certainly going to be both.
Also, the Feywild isn't associated with good, it's associated with the positive energy plane in old traditions. Likewise, the Shadowfell isn't evil, it's associated with the negative energy plane. Positive energy is life-until it overloads you and you explode-and negative energy is death, but neither is good or evil.
Although I an uncertain if the model still applies, that was the general rule before.