For those of you who have read the comics, you know that the Civil War storyline might be bad news for one of our favorite heroes. For those who haven't.... just know that the golden rule of Marvel is "nobody stays dead, except for Uncle Ben". But Captain America aka Steve Rogers seems totally unbreakble. It's a problem with superhero stories that every comics creator has run into: if the hero can't die, then the story has no tension. If there's no risk, the reward of victory at the end of a battle feels unearned. This is a huge challenge with Captain America in particular, because his origin story in the MCU rests on his ability to avoid certain death. So, if Marvel really is going down this path, they're going to have to find their version of kryptonite for some of our favorite characters. Give them a challenge that seems insurmountable, and give us a reason to be on the edge of our seats. They wouldn't need to look too far...
These pups are serioudly brutal. Their teeth can rip through metal and kevlar, and they will cheerfully ignore your screams of terror as they tear out your jugular. It's not personal, it's how they were trained. The teeth are usually replacements for the real thing, because the dogs have been trained to bite. HARD. And if that's not terrifying enough, remember that dog bites are extremely dangerous for humans. Their saliva is full of bacteria, some of which might be ambitious enough to give even Cap's super-serum enhanced immune system a run for its money. That is, if the dog didn't manage to chew him up first.
Stranger things have happened in the Marvel universe. Just hear me out: we had mind control in Agent Carter being used to trick people into walking into traffic. We saw in Age of Ultron that Steve is just as vulnerable to mind manipilation as everyone else. But why not take it one step further- TIME manipulation. Steve wasn't always a giant specimen of human super strength. Back in the forties he was scrawny, colorblind, asthmatic, and prone to catching every illness known to man. Just turn back the clock on his biology and hit him with a gnarly strain of the flu.
The nuclear option
I should probably have mentioned this earlier, but I don't actually WANT one of my favorite heroes to die. Dear Steve: if you're out there reading this, please stop jumping out of planes without a parachute and letting your best friend beat you to a pulp. I don't know where you got that deathwish, but you need to put it back where you found it bro. That being said, a nuke would probably kill him. They tend to obliterate everything around them, and irradiate everthing within several miles. The temperature of the blast alone can reach up to the tenths of millions of degrees. That is... extremely hot. So, if you're not worried about civilian casualties, and you really need to obliterate any trace of a national icon, one well-aimed nuke would probably do the trick.
Maybe it's because I just saw Jurassic Park, but lately I've been revisiting my childhood fear of velociraptors. Did anyone else have nightmares about these jerks of the dinosaur kingdom? I don't think just one would be able to take Cap down. But Steve is stubborn and doesn't generally think things through, which is why he has a habit of jumping into situations unprepared and without backup. So it might be possible to lure him into a dinosaur-filled trap of some kind.
A black hole
Because every Hydra agent just has one of those, right? This might seem a little excessive, but please remember that Captain America sustained at least three bullet wounds, multiple neck-snapping punches from an angry brainwashed assassin with a metal fist, and one deep dive in the Potomac without oxygen. And that was just one movie. So if all else fails, you can probably throw Captain Deathwish into a black hole, and just hope he doesn't come crawling back out.