It is very probable that the "horrible" bogeymen are actually some form of a group of benign creatures tasked with keeping the world safe from an alien invasion of Armok.
Here's why.
1) Bogeymen can talk, open doors and use tools. They have souls and dreams (you can see all that in DFHack). Most of them will dream of world peace.
2) Bogeymen will only strategically target the adventurer. Sure, they'll also exterminate anyone who is too close to the adventurer during the Cackling, but there's more to it. Proof to this is that while elven retreats and dwarf hillocks provide no protection against bogeymen, they still exist. If bogeymen were ubiquitous and attacked everyone, surely the elves would not survive.
3) Adventurers are posessed by Armok. Their internal states are completely overridden by a hostile alien force. While the possessed can cry in horror or be paralyzed by fear (watch your adventurer have eyes covered in tears as you murder children), their posessed body will pay no heed to those emotions. Armok is in full control.
4) Adventurers are evil. They go around and kill stuff and do all kinds of murderous things. Adventurers of any race or faction will have no problems butchering sapients even if it is unthinkable in their civ ethics.
5) Fortress Dwarves are a weaker form of mass posession. This is why bogeymen try to kill everyone else in adventure mode: they think that posession is infectious and are trying to prevent an Armok Base from taking root.
Long story short, bogeymen are DF's XCOM or MIB.