The Reason Marvel's Strongest Hero Didn't Beat The King in Black
Warning! Spoilers for King in Black: Thunderbolts #3 below.
Marvel's strongest hero, the Sentry, could have made a major dent against the King in Black but there was a reason he didn't ultimately beat Knull: He didn't blow himself up. The Thunderbolts have come up with one of the most insane plans ever to beat Knull - and one of the villains wonders why the most powerful hero in the universe couldn't have detonated himself and saved the day.
Knull, the ancient God of Symbiotes, has taken over the Earth and most of the galaxy, covering everything in darkness while unleashing his own army against both heroes and villains. While some heroes have been turned by the King in Black, the Kingpin reassembled (via threat) a new Thunderbolts squad to face the cosmic being and clean up the streets of New York. The group, which features Taskmaster, Star, Mister Fear, Batroc the Leaper, and a few other new recruits, have recovered Sentry's body after he was ripped in half by Knull and intend on using it as a weapon.
In King in Black: Thunderbolts #3 by Matthew Rosenberg, Juan Ferreyra, and VC's Joe Sabino, the Thunderbolts have recovered both halves of the Sentry's discarded body. As they make their way deeper into Manhattan, Star asks Taskmaster what their plan is now that they've got the heroes remains. After reminding her that they're going to explode him and unleash his "power of a thousand exploding suns," Star asks Taskmaster why Sentry just didn't kill Knull when he got the chance. Taskmasters response? Probably because he didn't want to blow himself up.
To be fair to the Sentry, he might have actually been willing to sacrifice himself if it meant killing the King in Black. With the powers of thousands of exploding suns, he's the strongest hero in the Marvel Universe - albeit, one that's extremely unpredictable. The Avengers prepared as best they could to deal with Knull's arrival on Earth and he shocked them by easily dispatching the most powerful hero on Earth. If given the second chance, knowing Knull's true strength, he probably would unleash his full powers in order to save the planet.
However, not knowing he would die at the hands of Knull, the Sentry greatly underestimated his power. The only way he could defeat the King in Black - and the plan might have failed entirely anyways - would be sacrificing himself, which obviously he wouldn't commit to ahead of time. But the Sentry was unaware of Knull's true powers and wasn't afforded the chance to detonate himself and possibly stop him, dying instead. There was no way to predict King in Black's attack - so Marvel's most powerful hero isn't at fault at all, even if he could have killed him. King in Black: Thunderbolts #3 is in comic stores now.