Be sincere will you return back to the address that mistakingly sent you 100BTC?
Well, the normal response would be not returning back such huge amount. I won't be a hypocrite, I might probably do the same thing and might sound foolish, but I would probably engage some of it into gambling to earn more, though there is a higher tendency to lose.
You may engage into investment than to risk in gambling. Why so?
Still it could happened in many circumtances if that happened it was not your fault and not your mistake and you have no responsibility to give back the bitcoin that you recieved.But i guess it will never happened if your going to send a whole 100 bitcoin they will be extra careful if they are going to send that amount and also will be probably not going to send a whole 100 bitcoin.