Warning: SPOILERS below for Thor: Ragnarok!
In a movie that was wall-to-wall laughs and surprises, it was one of the most surprising moments and one of the biggest laughs in Thor: Ragnarok: Fenris, the giant undead wolf of Hela the Goddess of Death (Cate Blanchett) was charging down the bridge towards the helpless people of Asgard. Despite shooting at Fenris repeatedly, the Valkyrie (Tessa Thompson) couldn't kill the enormous wolf. That was when Bruce Banner (Mark Ruffalo) made a fateful decision. Telling Valkyrie that everything was going to be okay now, Banner flung himself out of their spaceship, plummeted downward and... splattered himself on the bridge.
Banner was supposed to turn into the Hulk, which he did a few moments later. Audiences needed the few seconds to gather themselves from the shock of the gag before Hulk emerged to grab Fenris by the tail and have an epic fight with the giant wolf. However, Banner's hilarious bad landing still left many fans wondering if Banner actually died.
Banner wasn't trying to kill himself. It was clear from his intent that suicide wasn't his plan - Banner was planning to turn into the Hulk and make the transformation in mid-drop. Instead, somehow he mistimed his turn into green and belly flopped onto the hard bridge. We do hear him subtly whimper in pain, so no, Banner was not dead upon impact. And he didn't die. In fact, according to his own words and experiences, Banner can't kill himself.
Remember our first time meeting Ruffalo's version of Bruce Banner in The Avengers? Natasha Romanoff (Scarlett Johansson) tracked Banner to Kolkata, India, where he had managed to go over a year without Hulking out. Banner told Natasha that he actually did attempt suicide at one point:
I tried! I got low. I didn't see an end so I put a bullet in my mouth, and the other guy spit it out.
Banner literally can't off himself because the Hulk won't let him. As much as Hulk dislikes himself as Banner, and vice versa, the Hulk knows he can't exist without Banner. The stress of death or near-death on Banner's system instantly turns him into the Hulk as a defense mechanism - and the Hulk's imperviousness and ability to heal benefits Banner as well. This isn't to say the Hulk can't be killed, but from Banner's own account from his own previous attempt, the Hulk simply won't allow Banner to die.
Related: Thor: Ragnarok Wastes Bruce Banner
Instead, Banner's new worry is what happens to him if Hulk doesn't want to transform back into Banner. Bruce was horrified to learn that he remained as the Hulk for two years on the planet Sakaar - and Hulk got smarter during those two years. Banner feared the next time he transformed into the Hulk, Banner would disappear forever; he'll be "locked in the trunk" and replaced permanently by the Hulk. So turning into the Hulk on Asgard was a huge sacrifice for Banner, but he knew the Hulk's power was desperately needed by the Revengers.
If Banner does 'die', it'll be because Hulk won't let him come back now that Hulk is back behind the wheel. And who knows how long Hulk will remain the Hulk this time? Avengers: Infinity War will be the next time we see Hulk, and hopefully Banner as well.