What kind of man did Draco Malfoy become?
I don't think there were enough clues in the books to answer this, but I also know that there have been hundreds of times when J. K. Rowling has given talks and interviews and talked about her characters, so maybe she's said something about this.
Even though the Malfoys have behaved contemptibly at times, they do draw the line. Draco couldn't kill Dumbledore and they walk away from the Death Eaters toward the end, even though it might not have been safe to do that in the long run.
Is there anything that tells us what kind of man Draco Malfoy grew into? I think the last scene makes it clear he and Harry are at least able to acknowledge each other without hatred, and might even keep in touch, but that's all we see.
Has anything else been revealed about what he's like as an adult?
JKR has said that Draco was "sobered" by his experience as a Death Eater, and that he does not owe Harry a life debt. Draco marries Astoria Greengrass, the younger sister of Daphne Greengrass, Draco's classmate. They have one son named Scorpius Hyperion Malfoy, who JKR has said has a lot to overcome, not the least of which is his name (she meant Malfoy; I thought Scorpius was worse than Malfoy ;). Draco is never a "nice man." He does acknowledge Harry in the Epilogue with a curt nod; JKR says that under no circumstances could Draco and Harry be friends. Here's a funny quote about Draco:
Q: Does Malfoy owe Harry a debt?
JKR: That's a great question and a lot of people wanted to know that. When Dumbledore said to Harry, Voldemort won't want a close associate who is in your debt, I wasn't implying by that there was any kind of magical bond there. It was more that Dumbldore's extensive wisdom and knowledge of human nature, he knew as Harry later thinks in book seven, he knew that Pettigrew would react a certain way to having saved his life. ... He's weak, fundamentally weak. Pettigrew is a very weak character. He's not someone I like at all. He's a weak person and he likes to gravitate to people who are stronger. Dumbledore is right. Pettigrew had an impulsive mercy... would Malfoy be in Harry's debt? I think the very worst burden Harry could have put Malfoy under was this one, that Malfoy has to feel any kind of gratitude. So I tried to show that slightly in the epilogue when they look slightly at each other and there's a, "Hi. It's so embarrassing, you saved my life. No one will ever let me forget it." I think, does he owe him a debt, probably not. I think Malfoy would go back to being an improved version of what he was but we shouldn't expect him to be a really great guy any time soon.
Yeah, I don't see Draco becoming a humble boy-scout after the events of the Battle of Hogwarts. People can change, but let's not stretch it too far in Draco Malfoy's case. I'd say he mostly ended up somewhat like a slightly improved version of his father Lucius, with the added bonus of a little respect for Harry.
J. K. Rowling's Draco entry on Pottermore, added December 2014, directly addressed your question.
The biggest revelation was that he became less of a pure-blood fanatic:
The events of Draco’s late teens forever changed his life. He had had the beliefs with which he had grown up challenged in the most frightening way: he had experienced terror and despair, seen his parents suffer for their allegiance, and had witnessed the crumbling of all that his family had believed in. People whom Draco had been raised, or else had learned, to hate, such as Dumbledore, had offered him help and kindness, and Harry Potter had given him his life. After the events of the second wizarding war, Lucius found his son as affectionate as ever, but refusing to follow the same old pure-blood line.
Draco married the younger sister of a fellow Slytherin. Astoria Greengrass, who had gone through a similar (though less violent and frightening) conversion from pure-blood ideals to a more tolerant life view, was felt by Narcissa and Lucius to be something of a disappointment as a daughter-in-law. They had had high hopes of a girl whose family featured on the ‘Sacred Twenty-Eight’, but as Astoria refused to raise their grandson Scorpius in the belief that Muggles were scum, family gatherings were often fraught with tension.
Also, he seemingly mellowed out a bit, leading a life of leisure instead of working to overturn the forces of light or establish dominion over Muggles, like Lucius did:
I imagine that Draco grew up to lead a modified version of his father’s existence; independently wealthy, without any need to work, Draco inhabits Malfoy Manor with his wife and son. I see in his hobbies further confirmation of his dual nature. The collection of Dark artefacts harks back to family history, even though he keeps them in glass cases and does not use them. However, his strange interest in alchemical manuscripts, from which he never attempts to make a Philosopher’s Stone, hints at a wish for something other than wealth, perhaps even the wish to be a better man. I have high hopes that he will raise Scorpius to be a much kinder and more tolerant Malfoy than he was in his own youth.
There's a bit more information about Draco revealed in Harry Potter and the Cursed Child
He had a son, Scorpius with whom he has a somewhat strained relationship. There were persistent rumours that his son was the illegitimate offspring of Voldemort.
His wife died sometime during Scorpius' early childhood. Her death seems to have affected him deeply.
DRACO: Astoria always knew that she was not destined for old age. She wanted me to have somebody when she left, because . . . it is exceptionally lonely, being Draco Malfoy.
His wife was not a Death-Eater. His father did not approve of the match but he married her out of love.
He maintained his father's collection of dark magic memorabilia.
In the Cursed Child, it says Draco and Harry look at each other as friends for the first time. He also stands up for Harry and Hermione in the Cursed Child. Let’s also not forget he apologized to Ginny for messing up her kitchen. Finally, Harry’s son and Draco’s son become best friends. Yeah, Draco definitely turned into a nice person.
No one can say Draco was still bad because it is not true. There was a lot of proof on this.
Well, Draco certainly DID become a much better man, considering that he was a death eater at one point. But he never really became "A nice man". Some people say that Lily (Albus Potter's Sister) became friends with Scorpius, but I am not sure that that is confirmed by JKR. JK said that Draco taught Scorpius to become a much better kid than Draco himself was, but Scorpius is just a slightly improved miniature version of Draco, and Draco is a slightly improved miniature version of Lucius. I would think that some day, a Malfoy would not end up in Slytherin.
Such type of people generally evolve with great qualities as they have seen bad as well as good situations. And, I think Malfoy would have liked his goodness. In fact, there was a good in him. Bullying nature & support from dark side somewhat deviated him. But, he never actually turned to dark side like death eaters, that's why he didn't kill Dumbledore.
So, we can expect a better quality personality...
Well, going by the logic of the situation with Wormtail, one would think that Malfoy and Goyle would owe the trio a life debt for saving them in the Room of Requirement, but according to JKR they don't. I don't know why that is, maybe the magic works differently because everyone was in danger?
The one thing I never got was why Draco didn't marry Pansy, they seemed all lovey-dovey to me in the books, though I think JKR answered that question at some point also.
Harry didn't _save_ Peter's life as such; he _spared_ it. With Draco, it was simply a highly intense and extremely dangerous situation where Harry grabbed on and pulled Draco out—there was no honourable reasoning behind it, it was just plain fellow-man decency. I can see how that would make Draco grateful, but not really make him owe Harry a life-debt as such.
OK. So the end of the last book claims that Harry and Draco look at eachother and share some silent respect. I think that means that Draco will be good. He was starting to be good when he couldn't kill Dumbledore. It proved that he had a good heart. He wasn't evil enough to kill his Headmaster. I think that means something. And if he found a wife and had kids, he would want to be a good role model.