5. Tony Stark Leaves His Toys Lying Around (Iron Man 2)
One could probably argue Tony Stark cost innocent lives as soon as he became the heir apparent to his father's legacy; Stark Industries didn't make Mr. Burns' Old Fashioned, Good Times, Extra Chewy Cookies. Tony Stark's inherited fortune and legacy was built on the spilt blood of countless innocents. However, even after renouncing making weapons and reforming Stark Industries, Tony Stark has still opened a Pandora's Box by creating the Iron Man armor. While that in itself isn't a brainless act (quite the opposite), getting drunk and letting James Rhodes just waltz into your armory and steal the Mark II in Iron Man 2 certainly is. I mean for the love of God, smartphones now are coming with facial recognition security...and the man who created the most advanced technological weapon in the MCU to date couldn't even put biometric safeguards into it? Plus he not only doesn't have any kind of biometric security in them, he puts fully functioning arc reactors in them! In the MCU, that's the only thing that can power the Iron Man armor and Tony Stark conveniently could have made sure the only one available is the one in his chest. It's the equivalent of leaving a loaded gun in your house when you have kids; they may not find it today or the next, but they'll eventually find it and bad things will happen. Even Nick Fury and Black Widow make a comment about his brainlessness in the coffee shop, to which Stark brushes off and never answers...almost as if he purposely let Rhodes steal the Mark II. Mayhap so Iron Man would live on in some form after his demise.
However, the result is that it then allows Justin Hammer to modify the Mark II into the War Machine armor, which also lets Whiplash hack into it and take it over, and although it's never implicitly shown, some innocents had to die from collateral damage in that final battle as the compromised War Machine armor is chasing Iron Man around. We're not done with Mr. Stark...