Is it safe to show my old advisor my new paper? I've already graduated.  I wrote a paper related to my Master's thesis.  I am planning on sending it to my adviser to ask if I should ethically include his name as an author.  He seems to be ethical as far as I can tell, but in general is something like this dangerous to do?

Edit: I am wondering if showing your paper to other people before it is published is dangerous, I am worried about things like theft, even though he seems to be reputable, now that I am no longer connected to him academically.
If you work in the US or Europe, intend to complete the paper and submit it for review somewhere soon, and have records that show the development work that you've done on the paper, there's little harm in showing it to people that you trust. If you don't trust your former supervisor, then I'd wait until you've submitted it or even have an acceptance in hand. 

That being said, I'm wondering why you don't already know whether they should be a co-author and haven't already discussed the fact that you're writing a paper on which they might need to contribute. You should at least discuss this with them if there's any contribution of theirs in what you're writing up.