When it can speak and reason why would it be a "monster"?
What makes a kobold different from a gnome?
Imo the term "monster" is not one applied because of inherent racial traits but because of social stigmas. And those can change depending on the world or region.
Racial traits should often have social stigmas associated with them by definition. No different than ranchers in the real world killing wolves, regardless of whether wolves actually kill their lifestock.
If the Orcs have been attacking the City State of the Invincible Overlord for hundreds of years, why wouldn't Half-Orcs PCs be imprisoned or killed at the gates for being spys?
This is why the concept of evil monsters worked so well in earlier editions of the game system. Players often knew who they could attack with impunity.
When 4E came along and many different monsters were no longer evil (or no longer good) or even monsters by definition and when PCs could no longer Detect Evil, the game became more of a non-deterministic smorgasbord of creatures.
The players could determine the capabilities of a same race monster (knowing the difference between a Frost Goblin Cutter vs. a Corrupt Goblin Cutter vs. a Goblin Thorn vs. 200+ other types of goblins), with a monster knowledge check, but the players couldn't often determine the social stigma of an entire race of monsters because the monster could actually be played as a PC.
It doesn't make sense. Either all goblins are considered monsters and treated that way by most NPCs, or goblins should be able to willy nilly walk into any town unmolested until they actually attack. If the PC goblins are unmolested, then so should the NPC goblins.
Goblins shouldn't be given "inalienable human rights" just because the game system allows them to be played as a PC. They should be monsters first, attacked on sight by many different types of non-monstrous humaniods, and players playing them should be aware that this might be the case.
I don't think the game system by default should have racial political correctness tones where monsters walk the streets as if they were not monsters.
Tieflings are half-devils. They should be hunted down and slaughtered in a general default campaign setting.
Granted, a given campaign world could have Tiefling or any other race as benevolent creatures that most humans and other humanoids would trust, but that shouldn't be the default.
Races that are generally aggressive and non-benevolent should be considered monsters, regardless of their ability to speak and reason. For example, many PCs of any race.
