I think it's
not stealing for both cases because I think it wasn't a private property of some previous owner, it just happened he had the priv. keys, and now you also have. I tried to give a better explanation here:
https://bitcointalk.org/index.php?topic=1598043.0According to the idea that bitcoins exists in our "minds", what is happening is that you are able to communicate "to the air" and other people will hear that and they will arbitrarily decide to change their memory of the mental blockchain. So you can't be a theft just by saying some words to the air, while you're not taking any private property away from anyone. You are not more blamable than the people who decided to give their interpretation to your meaningless speech, and decided to ignore the (now) useless effort of the previous owner to re-spend his bitcoins.
Let's change the story: what if I'm walking in a mall, then I say something like "it's so could here", but some random guy hears me and thinks I said that I ordered him to go and kill random people. The interpretation he gives to what I said is his own business, I have nothing to do with his actions. But in the bitcoin case, the messages aren't "propositional" and there's not even private property aggression, anywhere. So in the bitcoin case I'm far more secure that it's not a wrong-doing.
Well, I think it's ok to think that way IF you consider that btc aren't priv. property. If not, if they are, then yes I would consider it a theft for both cases.
I very similar situation is when someone wrongly sends you some bitcoin, in a address you didn't expect any donation. Would you send it back? I mean, he could still just be donating random ppl some btc, but its very likely he is not. And if you consider it a donation, then (maybe) you can consider that everyone who participates on the blockchain are also donators to whoever have "their" private addresses.