OP has never expressed the intent to deprive, and so OP has not stolen the bag. OP's conduct may be immoral, it may open OP up to civil liability if the MAD passenger and/or BA suffer some sort of damages, but it certainly isn't criminal.

IMO, both BA and OP are at fault here. OP took the wrong bag and, as of yet, as not returned it. BA allowed the MAD passengers bag to be taken by a third party, and then when informed of this fact and the bags location, has taken no action to retrieve the bag.

And I find BA's actions much more egregious. If you operate an airline, you are going to have bags taken by people who are too busy to return the bag themselves. To the extent BA wants to hold OP responsible, BA should bill OP for the costs BA incurs. But BA shouldn't punish the innocent MAD customer for OP's inaction.