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The Monster Manual lists certain creatures as Evil. For example the Duergar, or Grey Dwarves, are Lawful Evil. Is a Duergar baby evil?

Here's the situation: A Duergar child is born to the king and queen of the Duergar realm, however the baby has the skin color of a Mountain Dwarf. Ashamed by their miscolored infant, and aware that this could lead to their downfall if it were to become known among their subjects, the Duergar king and queen leave their infant child in the wilderness to meet its fate. Unbeknown to the Duergar royals, the baby is taken in by a Mountain Dwarf clan leader to raise as her own child.

It so happens that this clan leader employs a wizard who has cast Glyph of Warding on each post of the clan leader's stone bed frame. The Glyph is set to trigger if an evil creature enters the bed chamber.

Question: Noting that infants, and particularly infants of humanoid species (as opposed to fiends and other monsters), regardless of race and heritage, and even regardless of their own fate, have the potential to grow up to be any alignment, and that infants are not yet capable of either evil thought or action; and also noting that in this specific case the baby would not yet have been influenced by her adopted mother, according to RAW would the glyph be triggered by this Duergar baby?

In other words: According to RAW are humanoid infants of an evil species necessarily evil as infants and would they trigger this glyph of warding?

tl;dr: With some exceptions (such as fiends, which are innately evil), alignment is a description of the moral outlook and attitude towards society of an individual. Racial tendencies are not hard and fast rules that apply to every member of the species. Creatures incapable of rational thought (including the baby in your example) cannot make moral choices and so are unaligned.

Let's take a look at the rules:

The first interesting sections are these:

A typical creature in the game world has an alignment, which broadly describes its moral and personal attitudes. Alignment is a combination of two factors: one identifies morality (good, evil, or neutral), and the other describes attitudes toward society and order (lawful, chaotic, or neutral).

and

For many thinking creatures, alignment is a moral choice. Humans, dwarves, elves, and other humanoid races can choose whether to follow the paths of good or evil, law or chaos. According to myth, the good-aligned gods who created these races gave them free will to choose their moral paths, knowing that good without free will is slavery.

and

Most creatures that lack the capacity for rational thought do not have alignments—they are unaligned.

So, morality is a choice, and a baby lacks the capacity for rational thought so a Duergar baby is unaligned.

For the sake of completeness:

The evil deities who created other races, though, made those races to serve them. Those races have strong inborn tendencies that match the nature of their gods. Most orcs share the violent, savage nature of the orc gods, and are thus inclined toward evil. Even if an orc chooses a good alignment, it struggles against its innate tendencies for its entire life.

I don't think a tendency towards evil would override the inability to choose.

Alignment is an essential part of the nature of celestials and fiends. A devil does not choose to be lawful evil, and it doesn’t tend toward lawful evil, but rather it is lawful evil in its essence. If it somehow ceased to be lawful evil, it would cease to be a devil.

Duergars are not fiends, so alignment is not an essential part of their nature.

To add to what was already said, for higher thought creatures, we can observe the tieflings.

In XGE, P.62, the tiefling heritage states that one parent was a tiefling or human and the other could be either a human, tiefling or devil. In that sense, regardless of how they came to be, one thing is clear. Infernal nature courses through their veins.

Despite that, the PHB says that tieflings aren't inherently evil, but tend to be chaotic in nature and often fall into neutral or evil alignment due to their rough upbringing.

With that, you can see that upbringing has more to do with alignment, but an inherently evil creature would be more prone to turn evil.