Sincerely, I do not know if I will return it or just keep it for my own sake. Imagine having a 100BTC to be mistakenly sent into your wallet is such a luck because there are thousands or even a million of different wallet which is uniquely different based on numbers and letter combinations and I am the one to have it. Maybe I will use it into an online casino to make some profit out on playing through its variety of games then, somehow send help to the needies to return back my luck by sharing some to them.
If I will receive 100BTC on my wallet due to mistaken identity on address, I would feel so lucky too because it maybe a blessing in disguise because out of nowhere, it was mistakenly sent to me out of too many combinations of addresses that exist. Maybe I will use it to start up a business to gain profit and somehow keep it for the good of my future. Maybe I can also try the way you have said about engaging into an online casino so that I am entertaining and enjoying myself while somehow earning while playing.