From the book's very beginning, Mr. U. N. Owen is always depicted as an enigmatic figure. This is actually an important detail within Christie's work: in fact, it is the central conceit around which the story relies. As we learn, U. N. Owen is a pseudonym used by one of the people on the island. No one knows who the murderer is; they only know that the murderer is one of them.
Early allusions to Owen tend to focus on his wealth. Thus, one can read excerpts such as "these Owens must be rolling in money" (Agatha Christie, And Then There Were None, chapter 1, section 6) or "Who were these Owens, he wondered? Rich and stinking, probably" (chapter 1, section 7). However, once the true nature of their gathering begins to set in, the mysterious Owen takes on a much more threatening quality.
Thus, U. N. Owen becomes perceived not in terms of his wealth (as he had been at first), but rather in terms of his insanity, attached to descriptors such as "lunatic" or "madman." At the same time, he becomes a source of intense paranoia: with the revelation that the murderer had been one of their party the entire time, the characters find they can not trust anyone, save those who have already been killed (and, as Christie's ending reveals, not even the dead can be fully exonerated, given the revelation of Owen's real identity).
As far as the second half of this question is concerned, keep in mind that we do learn Owen's identity at the end of the book. However, when discussing his motives, I would suggest you go back to the gramophone message that Owen had left. Ultimately, there is one driving commonality that has brought all of these various guests together: every member of this group has been responsible for the deaths of other people. In any supposition as to Owen's identity and motive, this detail needs to be taken into account.