Well, this nodeshell has been sitting here long enough for me to assume that its author is in no hurry to add a write-up. So, I thought I'd rescue it.
Since I have never heard of the word "antibuddha" before, I can only guess as to its meaning. Two possibilities come to mind:
The opposite of a buddha. Since a buddha is one that is fully awaken and liberated, free from samsara, especially one that obtained awakening through his own effort (as opposed to an arahant who obtained it with the help of a buddha), perhaps an antibuddha would be one that is fully immersed in samsara, one that is totally unenlightened, completely unaware whether of the sad condition his life is in, or of any possibility to change that condition.
One that is against a buddha. An example of this case would be Devadatta, the cousin of Gautama Buddha. Devadatta was quite an unenlightened dude (so the first definition would fit him well, too) who could not stand his cousin, the Buddha.
Not only did Devadatta not like the Buddha, he wanted him dead. He made several attempts to have the Buddha murdered (I guess he was too much of a coward to do it himself). Alas, he failed miserably in each attempt. While he hired the nastiest of assassins, whenever these came close to the Buddha, they were overcome by his presence and, instead of killing him, they became very attentive listeners and subsequently turned away from their assassinity (or whatever the word is).
Devadatta himself persisted in his hatred, which only goes to show that even a Buddha cannot push anyone into enlightenment (not that he ever tried to push anyone, he knew better). As Morpheus said in The Matrix: "I can only open the door. You have to walk through it."