So, goblins are not necessarily evil, beholders are not necessarily evil, lizardmen, dragons and so on, but snarls are?

How would you identify the difference?

Snarl killed innocents? So did Redcloak. He is just a mortal, but he caused death to countless people, even to his own.
You say Snarl can't control his actions because he is born from evil emotions, but Redcloak can - and he willingly chooses slaughter.

Kobolds become adventurers and join "not-evil-by-default-creatures" parties, vampires work as court advisors, bugbears are cruel to visitors but live far enough to not cause trouble, but goblins willingly choose to enslave and slaughter the whole huge city of humans!

But Redcloak isn't all this bad and deserves a chance, maybe through negotiations, but Snarl? No way, he is a bad guy, he must die!

No, where is the difference?
The Snarl is a unique creature, so much unique that he can't be compared to others?
If it is not judgement by race then what it is?

If one can negotiate with Redcloak, why negotiating with Snarl is impossible?