Warning: SPOILERS for X-Force #6
The X-Men's Beast is easily the most dangerous mutant of all - and he's a real threat to the X-Men themselves. Hank McCoy was one of the original X-Men, and in truth his greatest power is nothing to do with his various beastly mutations or his superhuman strength, speed and agility. Rather, the Beast is best known for his sheer genius.
In the real world, scientists tend to specialize. Comic books are different, of course, and as a result Marvel's greatest geniuses are remarkable generalists. As a result, Beast can pivot from maintaining the X-Men's Shi'ar tech to developing cures for the Legacy Virus, a genetically-engineered virus that targeted mutants. The breadth of his scientific achievements is probably second only to Mr Fantastic of the Fantastic Four.
But there's a massive problem with Beast; simply put, he doesn't have very good judgment to go with his genius. Beast is cocky and arrogant, convinced that he operates at least five moves ahead of everybody else. Because he assumes he has greatest insight than his peers, he frequently takes matters into his own hands, and has a history of making the most tragic mistakes. The first of these was when Beast thought he'd created a cure for his mutation, and injected himself with the serum he'd fashioned; it wasn't actually ready for human trials, and instead gave Beast his familiar furry appearance. The mutation appears to be progressive, meaning Hank McCoy's appearance continually changes; he fears that it may ultimately cost him his humanity, transforming Hank into a true Beast.
Unfortunately, humility is not one of the Beast's many talents. Rather than learn from this mistake, he's instead repeated it, so confident in his own judgment that he's taken the most insane risks. As an example, in the aftermath of 2012's Avengers Vs. X-Men event Beast became convinced Cyclops was leading the mutant race toward a civil war. He came up with the idea of plucking the original X-Men from the past and bringing them to the present day, somehow convinced that this would put matters right. Instead, Beast nearly broke the entire timeline, finding himself unable to send these "All-New X-Men" back. "I thought I understood the fundamentals of time travel," he admitted in a rare moment of honesty. "But they speak to an understanding of the universe far greater than humankind's collective experiences." His actions led to no less a being than the Watcher, purveyor of the Multiverse, appearing to express his utter disgust with Hank McCoy.
Even that assessment - by a near-omniscient being no less - didn't seem to shake Beast for long. In this week's X-Force #6, he's at it again, convinced that he can take action without supervision or even a second opinion. This time round, Beast has correctly realized that human scientists had inadvertently created a new race of post-human bio-Sentinels. He took drastic measures to prevent these bio-Sentinels taking the next step along their evolutionary chain, which he feared would turn the Earth's entire biosphere into an anti-mutant weapon. Shockingly, Beast conducted a lobotomy on one of the people infected with the bio-Sentinel strain. What he didn't know, though, was that he'd unwittingly unlocked said next step of evolution. Beast may have actually doomed the entire mutant race.
During 2013's "Battle of the Atom" event, SHIELD boss Maria Hill fumed that Beast was the mutant threat who kept her awake at night. "Henry McCoy is always doing whatever he wants whenever he wants," Hill seethed. "People always complain about Wolverine... but Wolverine holds to a code. He's a samurai with a code. A messed up code, but a code nonetheless." In contrast, Beast has no real code at all, bar a disturbingly unshakable confidence in his own judgment. And that makes him more dangerous than any other member of the X-Men.