"Yaroo" on the end of bakayaro makes the "baka" part a bit more vulgar. It is like calling someone a moron, idiot, ***hole, if you get the drift. If I am not mistaken, the "yaroo" is from the verb yaru which is a very familiar form of the verb suru (which means "to do" in Japanese). The familiar form, and the conugational tense implies that the person referenced is willfully being a jackass and making a spectacle of himself. The vulgarity is highlighted by verbally calling attention to the person for his actions, and potentially bringing him a loss of face due to your implication. It is a common way to put someone down or 'dis' them for their actions. You might also like to know that the "ba" comes from "horse" in the Chinese characters, and "ka" comes from "deer", although the common written form today is to use the simplified hiragana writing system, rather than the Kanji form. However, because of the meaning of the two characters, the statement also indicates that the person is like the misbegotten offspring of a domestic and a wild animal, which makes him utterly useless and ignorant.
Bakayaro! Itsumo, itsumo osoku made nondeiru. kesa, moo juuji sugita, okinakatta kara shigoto de kubi wo kiru hazu ja nai ka?
Dumb butt! Always, ALWAYS you are drinking until it's late. Then this morning it's already past ten o'clock and because you didn't get up, shouldn't they fire you at work.
(literally kubi wo kiru = cut your neck)