Howard Hawks once said a great movie has three good scenes and nothing bad—The Untouchables certainly delivers on that front, boasting one of cinema's most memorable sequences, which draws inspiration from Battleship Potemkin and has since become legendary in its own right. This film by Brian De Palma contains several other enjoyable moments, and it’s hard to find fault with any of them; although, a couple of scenes are baffling, even among the better ones. The bat sequence is particularly entertaining, but I'm still unsure about the identity of the unfortunate fellow Al Capone (Robert De Niro) brutally assaults, or why he was subjected to such treatment. I’ve seen some explanations online—including what I suspect might be the reason—but none seem quite right. It's just as improbable that Al Capone would personally kill a random person as it is that Eliot Ness (Kevin Costner) would shove Frank Nitti (the wonderfully sinister Billy Drago) off a building—Nitti died by his own hand long after these events transpired. Still, De Palma includes a detail referencing history: Ness shoots holes in Nitti’s hat; Wikipedia notes that Nitti's fedora was penetrated by a bullet during his suicide. I usually get upset when historical films take liberties with the truth, but The Untouchables isn’t meant to be a documentary—it’s more like a fanciful story featuring a folk hero (and Costner embodies that quality perfectly) and an iconic villain. That's as good of an explanation as we are likely to receive regarding the bat scene; Capone acts simply because he can. What matters most is that despite these alterations, the film maintains a strong sense of morality, adhering to the principle that “the law is the law.”

Source on TMDB