Often in comic books, there's a thin line between good and evil. That line also tends to be liberally crossed more often than not in the Marvel universe. The result was a lot of villains ending up getting recruited in the good guy pool, which in this case, is the Avengers. In fact, the number of villains in Marvel who became Avengers is quite high.
Simultaneously, there are also some bad eggs in the superhero roster of the Avengers. Many of them have betrayed the Avengers time and again and even caused more damage than some supervillains could. So, in case you're still not convinced that Stark or whoever's recruiting the Avengers need to straighten up their recruitment criteria, here are five villains who once became Avengers and five heroes who betrayed them.
10 SCARLET WITCH/QUICKSILVER (VILLAIN)
These biological twins are two of the most famous (or infamous) mutants in Marvel and you really can't have one without the other. They tend to stick together... tightly... in many ways. Initially, they were evil mutants serving under Magneto's Brotherhood of Mutants which directly opposes the X-Men's ideals and goals.
Eventually, the two of them got clean and were even recruited in the Avengers. Heck, they even debuted in Avengers: Age of Ultron. Even so, they have a tendency to be loose cannons, especially Scarlet Witch (Wanda Maximoff). Time and again, she can easily turn back into a formidable villain whose pretty much on the same level of power and instability as Jean Grey.
9 VISION (HERO)
Speaking of Scarlet Witch, she's not the only who has a bad track record in the Avengers. Her once husband, Vision, despite being a heroic robot, has been a very bad boy at least once in his tenure as an Avenger. Being an extremely (and even annoyingly) logical robot, Vision has a penchant to be a softcore Ultron.
At one point, he linked up with a supercomputer from Titan (Thanos' home planet) and it filled his head with ideas of extreme measures for protecting the Earth. Vision even thought that in order to best protect Earth, he alone must run it. When his plan proved to be somewhat of a failure, Vision began using drastic measures and took over every computer on the planet, particularly the ones which controlled Earth's weapons.
8 SANDMAN (VILLAIN)
Sandman is definitely not the first Spider-Man supervillain to get a bigger role outside of his respective comic book world but he's one of the few lucky enough to be an Avenger-- a good Avenger. Technically, he was a reserve member but it still counts. It began when Fantastic Four's The Thing ran into Sandman and the two had a little heart-to-heart about his going clean.
Sandman, swayed by The Thing's rock-solid, er, morals, eventually caved and became a reformed Spider-Man villain who helped the webslinger from time to time. Afterward, Captain America invited Sandman to be an Avenger after the latter received a presidential pardon. It's too bad this didn't last and Sandman went back to his villainous ways; it's probably because he was way underutilized as an Avenger.
7 NAMOR (HERO)
Marvel's resident Aquaman (though Namor actually existed two years earlier than Aquaman); like the DC superhero, he doesn't get along too well with other metahumans. In fact, he frequently had clashes and wars with another Avengers mainstay, namely Black Panther and his kingdom of Wakanda.
Even so, he was an Avenger... for a time. As soon as the Earth's mightiest heroes had to fight the X-Men in the Avengers vs. X-Men storyline, Namor was quick to turn his back on the Avengers. He then sided with the X-Men and proceeded to destroy Wakanda with a giant wave as it was where the Avengers were hiding. The Avengers survived but Namor's actions caused another major rift in his relationship with Black Panther.
6 SABRETOOTH (VILLAIN)
Yes, he was, Wolverine's hairy nemesis and long-time attitude rival, Sabretooth was once an honorary Avenger. Sabretooth's rehabilitation as a good guy came in the form of an overdone attempt to reverse the Red Onslaught's (Red Skull) moral compass. Turns out, the attempt was too strong; this turned all surrounding supervillains into heroes while consequently making some of the good X-Men evil.
As a result, Sabretooth had to join the Avengers in correcting their reversal. It was successful but Sabretooth was spared the reversal attempt thanks to Iron Man; he remained an inverted good guy. After that event, Sabretooth went on to join the Avengers Unity Division.
5 SENTRY (HERO)
Given how stable Sentry is most of the time, this one shouldn't really be surprising. Sentry's betrayal of the good Avengers happened after Norman Osborn took over the United States and assembled his own pseudo-mightiest heroes roster called the Dark Avengers. Osborn, aware of Sentry's condition, bribed the yellow guy into joining with the promise of curing him.
To make matters worse, Sentry is way too powerful even for the likes of Thor or Hulk. He became instrumental in destroying Asgard as a Dark Avenger. Of course, Thor got angry, hurled a few hurtful words and some lightning in Sentry's way, then finally threw Sentry into the sun.
4 VENOM (VILLAIN)
Over the years, Venom has become more of an antihero than a supervillain or sometimes tiptoes the thin line between the two. Apparently, the alien symbiote learns plenty depending on their host, so when it finally traded in Eddie Brock for Flash Thompson (Peter Parker's high school bully), Venom became a full-fledged hero.
His name was also changed to Agent Venom; Flash was a US military soldier who lost his legs in Iraq. He was given the Venom symbiote so he could continue his special ops missions. Eventually, Agent Venom managed to join the Secret Avengers, earned Hawkeye's trust and even became a pivotal member of the team.
3 CAPTAIN AMERICA (HERO)
This right here is one of the top anime-- sorry, comic books betrayals of all time. The writers at Marvel went deranged with a certain storyline titled Secret Empire. They made Stever Rogers (Captain America) do a 180-degree Hydra overhaul. It wasn't his fault, though; it was a sentient Cosmic Cube which rewrote Rogers' history so that he became a Hydra sleeper agent instead.
The result was catastrophic. Hydra Cap took over the US and even managed to kill some of his several of his friends. Thankfully, Hydra Cap was defeated thanks to the combined efforts of his closest friends (Falcon, etc.). Still, the storyline was one of the books; it made the comic book industry (and the internet) explode, especially when the once pristine Captain America started uttering the most impossible combination of words to ever come out of his mouth.
2 RED HULK (VILLAIN)
The Red Hulk was one mean pain machine when he was first introduced into Marvel. One of his most notable victories was against the She-Hulk and her own band of female Avengers and superheroes. No one really found out that he was General Thaddeus Ross even when the Hulk (Bruce Banner) finally defeated him.
After his capture, Captain America enlisted Red Hulk in the Avengers against the fight with The Hood. In fact, Red Hulk himself even snagged himself a Power Stone and became instrumental in the fight. His immense help solidified his position in the Avengers though his teammates still didn't trust him.
1 HULK (HERO)
The Hulk was never really one of the most trusted or welcome members of the Avengers and he knew it just as his alter ego, Bruce Banner, did. As such, the Hulk quit the team soon after the battle with the Space Phantom back in Avengers #2 (1963). Namor then took this opportunity to hatch a revenge plan with the Hulk since they technically have similar sentiments against the Avengers.
The two executed their plan and even nearly won. However, the battle proved to be a little too stressful for the Hulk when it reached its critical moment. This caused him to transform back into Banner and flee. Consequently, the tide turned on Namor (ba dum tss) and he had to retreat without the Hulk's help.