Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-24398707-20140108222502/@comment-19765459-20140108223402

Peter seems to know a lot more about the occult than anyone else (including Deaton). His tone of voice and facial expressions when speaking about his sister Talia make it clear that he always planned to kill her and become the Alpha, and the fire merely changed the plan (and the target). His ability to use his special Alpha powers exceeded those of any other werewolf we've seen (including Deucalion) and he has even retained some of them (like mind reading) since reverting to being a Beta. Also of note, he knew about the Nemeton, and was already there waiting when Jennifer tried to get healed again.

Finally, he is sticking around Beacon Hills, even though he clearly did not regard Derek as his Alpha, and does not regard Scott as such either.

So I think that he is hoping to gain control over the magical power of the Nemeton and acquire abilities beyond anything even Deucalion imagined.