Some superheroes have a code against killing, like Superman or Batman. Captain America is one of a few who has killed and wouldn't shy away from it as an option. It’s not something Steve Rogers, the man behind the shield, revels in. Yet if it's a viable option, he'll take it. There are plenty of golden age comics where Cap toasts one bunch of anonymous Nazis or another. The MCU continued this trend with different aliens, henchmen, and other goons getting finished off by the Captain.
Still, it is more likely to find scenes where Captain America defeats a villain non-lethally or sacrifices himself instead of killing others. Yet, with a little digging, there are a few named characters who lost their lives at the hands of the first Avenger.
6 Machinesmith
Originally a Daredevil villain, Starr ‘Machinesmith’ Saxon used his knowledge of robotics to frustrate one bunch of heroes or another. He got involved with The Fantastic Four and the X-Men among others. Then he took on Captain America in Captain America #249. He took his consciousness out of his physical body and sent it from one robotic double or another to avoid death. But in safeguarding his life, he ended up growing tired of it and sought a way to end it.
He came up with a plan to commit suicide by a superhero. He takes Captain America on with multiple android versions of himself, while leaving the central computer visible. Thinking he could trap Machinesmith in one body, Cap destroys the computer to keep it from transferring himself across his duplicates. Instead, he destroyed the last remaining source of Machinesmith’s consciousness, killing him off for good…or for a few years anyway.
5 Baron Blood
In Captain America #254, Cap went to the UK to take care of a vampire problem. An old friend of his, the British superhero Union Jack, had some family trouble when his brother, Baron Blood, became a vampire. The Baron fought against Cap, Bucky, and Namor back in World War 2. Now, Blood was back, and Cap had to join forces with Joey Chapman, the new Union Jack, to stop him.
The duo managed to pin him down, but they had no way of keeping him down. Being a vampire, the Baron would just shrug off any injury or break free from any stronghold. The only way to stop him for good was to kill him. Reluctantly, Cap picked up his shield and lopped the creature’s head off with the edge.
4 Magneto
This one is more of a team effort. In the Marvel Zombies universe, Captain America got a promotion to Colonel. Not that it saved him when the zombie apocalypse took place. Eventually, every living being on Earth either became a zombie or was eaten by the zombies. One of the few living supers, Magneto, helped the Ultimate Fantastic Four and some human survivors escape back to their universe.
Then he became the only person left to face off against Colonel America and the rest of the undead heroes. He managed to lop the top of the Colonel's head with his own shield, which annoyed him (“If we catch him - when we catch him- I get double rations!”). Eventually, Hawkeye and Wasp got the better of him, and Magneto became chow for them, the Colonel, and every other hero in the area.
3 Red Skull
If regular Nazi troops got the fatal end of the wedge, surely Captain America’s arch-nemesis should get the same. The two did have a climactic fight in Captain America #300, where Cap sought to avenge his fallen friends. However, Red Skull’s death there came from poisoning both himself and Cap. Even as he lay dying, Cap refused to finish him off. It's a noble and typical comic book way of keeping a hero from taking the lethal option.
What If? #26 had something much more conclusive. Captain America becomes President of the US, goes public with his identity and provides the US with a new, clean source of energy through a solar satellite. Unfortunately, he gets caught by the Red Skull on a diplomatic visit, and he plans on turning Cap’s own satellite into an orbital laser to obliterate Washington DC. Cap breaks free, redirects the satellite’s coordinates to his location, and keeps the Red Skull occupied until it fires. So, Cap gets to sacrifice his life and kill someone at the same time.
2 Iron Man
Red Skull may traditionally be Captain America’s main villain. However, his most notable rival in the past decade and change is fellow hero Tony 'Iron Man' Stark. The two were just co-Avengers for decades. Then after Marvel released the Civil War event comic in 2006, the two have often butted heads over one conflict or another. Their most famous outing was in Captain America: Civil War, which cleaned up the comic's messier aspects into a clearer battle of ideals
It wasn’t until Age of X: Universe that things took a bloody turn. In it, the heroes got caught up in a government plot to exterminate the world's mutant population. During the conflict, Iron Man gets infected with a virus that traps him in his suit, slowly killing him as a result. When the Avengers decide to go against their mission and protect some mutant children, Iron Man’s suit takes aim at them against his will. With Stark’s permission, Cap puts him down with a rifle round to the back of his head.
1 Galactus
How could Captain America take on a giant, planet-eating entity like Galactus? Few people from any corner of Marvel's universe have gotten one up on him. The most famous time wasn't even in a fight, but with a threat. The Fantastic Four's Reed Richards threatened to use the Ultimate Nullifier to wipe out the entire universe if Galactus laid a finger on Earth. He decided not to call his bluff and left the planet alone.
So, what if someone dared to use the device? Enter What If? #32, where the villain Korvac kills off the Avengers and resurrects them as his pawns. The Watcher brings the most powerful forces in the universe to stop them, including Galactus. That sounds like overkill, but Korvac is smarter than he looks. He has Cap sneak into Galactus’ ship, get the Ultimate Nullifier, and use it on him. The godlike being gets atomized, as does Cap in the process. Steve Rogers may be done in the MCU, though Anthony Mackie might want to double-check future scripts for Sam Wilson in the future.