I think that Warcraft 3 is absolutely fine for an average 8 year old kid.
First of all - there is a bit of violence in the game. I'm talking about things like environment decorations including dead bodies, heads on spikes, blood, torture contraptions, etc. And like in most games of this type, there's some killing going on. That said, the game has a rather cartoony-fantasy art style, so all these elements are presented in a very non-graphic way plus as
@Nowow mentioned, they're showed from a distant camera, so I don't think they're should be an issue.
And if you're not sure - ask your kid if he knows what a "zombie" is. I'd wager a bet that he does and if so, he most likely has already been exposed to a lot more violence than Warcraft 3 has to offer, so the game won't shock him in any way.
Other than that, there's nothing else you should be worried about - no adult language, nudity, sex or any other stuff a parent might be worried about.
P.S. I went and checked - the game is rated T for Teen and is recommended for kids of 12+ years old if you're interested i knowing what the ratings are.