Not sure whether my book counts as "adult" or not

Adult content usually means sex-related. There might be restrictions on language and violence for children, but they are not referred to as adult content.

I think it depends on the tone of your book, rather than on what it contains. The Hunger Games is definitely YA yet there are tons of bloody scenes. Who is your target audiance?

In the US, adult content usually means explicit sex and swearing (most particularly the F word and S word, and slang language that refers to certain body parts--certain words will get a movie and "R" rating here, for instance). Rightly or wrongly, violence gets more of a pass, unless it's extremely graphic and gory, like loving, detailed descriptions of someone being eviscerated then strangled with their own intestines, maybe.

I'd read Smashword's policies on what constitutes adult, however, as they may have their own standards. There's no across the board rating systems for books in the US (aside from stories that are very specifically intended to be erotica or pornographic), so it's a matter of a particular publisher's or website's policies.

I'm not the world's biggest George RR Martin fan, but I have to give him props for understanding this distinction extremely well:

“I can describe an axe entering a human skull in great explicit detail and no one will blink twice at it. I provide a similar description, just as detailed, of a penis entering a vagina, and I get letters about it and people swearing off. To my mind this is kind of frustrating, it’s madness. Ultimately, in the history of [the] world, penises entering vaginas have given a lot of people a lot of pleasure; axes entering skulls, well, not so much.”

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I'd second the advice to check Smashword for their policies, but as long as you don't have explicit sex or strong language, you're probably fine.