*** Here there be spoilers, for the ending of Civil War (comic book)***
By now it's well known that the narrative that's going to form the crux of the upcoming [Captain America: Civil War](tag:994409) is the ideological battle between Tony Stark (Iron Man) and Captain America (Steve Rogers), divided on opinion over the introduction of a governing body to supervise the Avengers activities.
In the comic series from which the title takes it's name, the entire premise is based on this split between sides over the Superhero Registration Act introduced following a tragic catastrophe sparking years of building tension in the Marvel comics universe.
Since the Marvel Cinematic Universe is nowhere near as vast and established as the comics; the ideological struggle will still be the central theme but another angle has to be taken with other threats brought in from elsewhere. Civil War is going to pull the team apart but something has to bring them back together for Avengers 3: Infinity War, so it comes at no surprise to see the inclusion of some classic Marvel villains on the cast roster.
Brock Rumlow, A.K.A. Crossbones, is one of Captain America's main antagonists throughout his history. An super-buff super-villain, agent of HYDRA, mercenary, assassin and close subordinate of the Red Skull - Crossbones is a formidable foe. He's an expertly trained fighter and martial artist, an expert marksman with various weapons and he's skilled in the art of torture and brainwashing. Later in the comics he gains the ability to shoot flaming energy beams out of his face, however I doubt we'll see that power in his MCU counterpart (although, never say never...).
He was seen fighting Cap. and co in the Comic Con trailer (which hasn't been officially released yet). Now we already met this dude back in [Captain America: The Winter Soldier](tag:254973), played by Frank Grillo. The Winter Soldier left the HYDRA agent badly burned in the helicarrier crash, but he's back for round two revenge with Captain America in Captain America 3.
Why is he important?
Well, he kills Captain America.
In the comics, just after Civil War ends, Crossbones and a brainwashed Agent Carter shoot Cap when he's in custody, killing him. This does give us one of the most heartbreaking scenes in Marvel history, but like Tony says, it's not worth it.
I highly doubt this will happen in Civil War; they're certainly not going to kill Cap off completely when Chris Evans still has a film left on his contract, but if they decide to go the way of the comics (revealing later that he didn't actually die, but instead was slipping in and out of time and space) there is a possibility that Crossbones could take down the First Avenger.
Another classic Captain America villain enters the fray, this time in the form of Baron Zemo, portrayed by Daniel Brühl. Now, Baron Zemo refers to more than one character in the Marvel universe; it's a title passed down in a lineage from father to son, so there's no guarantee that the Captain America 3 Baron Zemo will be anything like that of the two most prevalent: Heinrich Zemo and Helmut Zemo.
A sadistic genuis, Heinrich was a Nazi scientist and creator of WMDs who Cap battled repeatedly during World War II. The red hood that he wears is permanently bonded to his face by Adhesive X, a substance he invented that fell on his face when he was attacked by Cap. After this happened he went a little bit insane.
Helmut Zemo, the son of Heinrich, also emerged as a super-villain opposing Captain America who he blamed for his fathers death. He also ended up wearing the hood almost permanently, as he fell into a vat of molten Adhesive X and was hideously scarred.
Why is he important?
In addition to the ongoing fight with Cap, Zemo also has a long history with Bucky Barnes (Winter Soldier). It was Heinrich Zemo that caused his apparent death during World War II and Helmut Zemo (who is jealous of him) has kidnapped and drugged Bucky to get him to commit unlawful acts.
Given from what we've seen so far, Bucky is going to be still under the affects of mind control and recovering details of his former life, so the presence of Baron Zemo could have dire consequences for him.
Okay, given that the Hulk (Bruce Banner) isn't appearing in Captain America 3: Civil War, it's hard to estimate how much of a threat General Thaddeus E. "Thunderbolt" Ross will be to the Avengers team, but if he's anything like we've seen so far he could cause real problems.
In the MCU so far we've seen Ross in The Incredible Hulk as an antagonist: he attempts to weaponise the Hulk transformation process, which can't be good for anyone.
Why is he important?
In the comic books he eventually becomes the Red Hulk in his efforts to defeat Hulk. He's not evil though, more like the poster child for "the road to hell is paved with good intentions". In Civil War Ross is now US Secretary of State, which affords him an awful lot of power behind those misguided intentions.