Do I need to see "John Wick" to understand "John Wick: Chapter 2"?
The 2nd film is "Chapter 2", and it lives up to the title, as it starts immediately after the first one ends.
Yes, there is an opening, pre-title, extended scene that has supporting-characters summarizing and expanding-upon the plot, events, and purpose of the first film. So technically speaking, you can watch "John Wick: Chapter 2" without having seen "John Wick", and you'll be given a cursory summation of where the story is at.
Plot-wise, there are some continuity throw-backs and relationships that you'll need to follow, but most of that is very blunt and very obvious to pick-up on.
Short answer: No. You can understand "John Wick: Chapter 2" without having seen "John Wick".
Do I need to see "John Wick" to (fully) enjoy "John Wick: Chapter 2"?
Personally, I don't care about guns, movie-murders, nor do I inherently enjoy violence.
If you do enjoy the specifics of guns and fancy reloading and aiming and movie-murders and mixed martial-arts, you might very well have different better or worse opinions, because they do introduce those things in the original and heavily emphasize (quadrupling-down on) all of them in "Chapter 2".
But what I enjoyed about "John Wick" was the artistry and the pacing. For a more "cerebral" and subtler take on similar topics, I whole-heartedly suggest you look for "The Rover" (http://www.imdb.com/title/tt2345737/). It's more satirical and philosophical, but has overlapping themes and plot elements.
So with enjoying "John Wick", for me it was about the craftswork of the shock-and-awe pacing of the stunts and actions, and the way the film presented itself as it was; nothing more and nothing less. It was surprising, intense, hilarious at times, and continuously shocking. Was it kind of ultimately stupid/pointless/vague ...? That's certainly debatable.
With "John Wick: Chapter 2", it takes on -- from my perspective -- this interesting role as being a far more literal continuation of the first film.
Without spoilers, everything that happens in the first film has now just finished happening hours if not minutes before the opening scene of "John Wick: Chapter 2". Because of everything John Wick just went through in the first film, he's now physically, emotionally, and mentally taxed. But again, he's thrown immediately into the events of the second film -- think: less humor, more struggle.
I would think you could enjoy Chapter 2 without having seen the original, but it's clearly at least an impending trilogy that is turning out more like a single 6 hour film. It's the classic kind of trilogy pacing. The first film is the setup and the surprise and wonder of a new universe. The second is like just a constant 2 hour battle with random, confusing plot jibberish thrown around as if we're supposed to be buying into the reality of the universe. And I'm fully expecting the third will be this dramatic coalescing of the first two, with this "ultimate" expression of the simple, basic truth ... don't fuck with John Wick. [Note this pacing with the original "Lord of the Rings" trilogy.]
It's by no means "too confusing", it's just that you're walking halfway in.