I can't say that your situation is stealing of bitcoin. Cause of stealing is doing something illegal for purpose. And in your case it was without purpose it's just happen like that.

It's not a secret anymore, because it is detrimental to the owner of the bitcoin. Something illegal is something that everyone can do harm or not get recognition from everyone

First miracle generated a new address and get Bitcoin impossible.You can said you have a bitcoin address and you get bitcoin somehow miracle.second if you get something illegally that must be illegal there is no confusion.So obviously you have to resend the bitcoin to the owner that would be good.

Let us take it this way, i have the address and the one to  use it and then suddenly i saw some btc on it, would you accuse me of a thief if i use the btc in my account that was created by me?  I can accuse you of something related to using my account without any permission then if you push that robbery thing to accuse me.  But on the topic, stealing is never been legal.  This is actually complicated and the best solution is adjustment.

if really you have the adress in your wallet so why hurry to spend the amount received if it is not the intention to rob.
so the answer to your question is yes i will accuse you of a thieft

well of course its illegal ..well to be honest u got to have a lot of knowledge if u wanna stell bitcoins ..dude dont ever keep thsi thing in mind ..because if u  start right now of steeling bitcoins ..then in later off u would start stealing other stuffs ..so buddy think of how can u earn bitcoins rather than stealing ..kudoos

of course it is illegal? how does a move when you take something that is not yours not be illegal? if you stole bitcoins then i think you would have big trouble with laws

yeah it is illegal and i think it is a really great thing because then there are less thieves who want to steal all of my money i hope i will never get my coins stolen

I'll bet one dollar if anyone manages to generate an address which already has BTC in it.

The probability to get struck by lightning 4 times in a day is bigger than that.