Bad births and not so Bad births? As we know there are lower realms mentioned in Buddhism, like the animal realm.

If you have heard the story of the "Alavaka yaksha/yakkha" (He was an evil being by birth)

You might also think "If beings like "Alavaka" can reach goals in the path are they considered lower because they are evil by birth?"

After all he is not the only one of his kind to reach goals of the path on the spot where they learned Buddhism.As to Suttas/Sutras these beings serve as soldiers in armies of Gods/Devas, so they clearly aren't some primitive things like our animals.These beings also have ridiculous strength and amazing abilities like shape shifting.  



So what i'm asking is...

Are they labeled Lower realm creatures because they are Bad or Evil from birth or is there any other reason?

(Please note that i am only asking about the Race/Species/Beings 
called  Yakkha or Yaksha)
This page which describes The Thirty-one Planes of Existence places yakkhas immediately above the human realm:

Devas of the Four Great Kings (catumaharajika deva) -- Home of the gandhabbas, the celestial musicians, and the yakkhas, tree spirits of varying degrees of ethical purity. The latter are analogous to the goblins, trolls, and fairies of Western fairy tales.

See also this Wikipedia article,

Yaksha (Sanskrit यक्ष yakṣa, Pali yakkha) is the name of a broad class of nature-spirits, usually benevolent, who are caretakers of the natural treasures hidden in the earth and tree roots. They appear in Hindu, Jain and Buddhist texts. The feminine form of the word is yakṣī or Yakshini (yakṣiṇī).
In Hindu, Jain, and Buddhist texts, the yakṣa has a dual personality. On the one hand, a yakṣa may be an inoffensive nature-fairy, associated with woods and mountains; but there is also a darker version of the yakṣa, which is a kind of ghost (bhuta) that haunts the wilderness and waylays and devours travelers, similar to the rakṣasas.
Yakṣas in Buddhism -- In Buddhist literature, the yakṣa are the attendants of Vaiśravaṇa, the guardian of the northern quarter, a beneficent god who protects the righteous. The term also refers to the Twelve Heavenly Generals who guard Bhaiṣajyaguru, the Medicine Buddha.

See also this (long) definition of Yakkha (as well as this page which contain back-story to the Alavaka Sutta).

The idea of a dangerous guardian reminds me of this other topic about a &quot;guardian&quot; that seems &quot;wrathful&quot; and/or dangerous: What is a wrathful Buddha?