Since the question doesn't specify in-universe, I'd like to add the out-of-universe answer to supplement the comprehensive in-universe answers so far.
Out of universe, the answer is that J.J. Abrams took the advice of Jony Ive, chief design officer at Apple, that a "more primitive"-looking lightsaber, which lacked precision, would look "more ominous". As The New Yorker reported in February of 2015,
...Ive once sat next to J. J. Abrams at a boozy dinner party in New
York, and made what Abrams recalled as "very specific" suggestions
about the design of lightsabres [sic]. Abrams told me that "Star Wars: The
Force Awakens" would reflect those thoughts, but he wouldn’t say how.
After the release of the film’s first trailer—which featured a fiery
new lightsabre [sic], with a cross guard, and a resemblance to a burning
crucifix—I asked Ive about his contribution. "It was just a
conversation," he said, then explained that, although he’d said
nothing about cross guards, he had made a case for unevenness: "I
thought it would be interesting if it were less precise, and just a
little bit more spitty." A redesigned weapon could be "more analog and
more primitive, and I think, in that way, somehow more ominous."
I think we can see from the film that Kylo Ren's lightsaber is indeed "more spitty" than any other we've seen, as noted in the question's mention of "stuff coming out of it". It certainly does suit the character of its reckless owner very well, and in the history of the film's production, credit for that concept seems to go to Jony Ive.