What do you mean use 100 words on your site?
You mean optimize your site for 100 words?
If you crammed 100 unconnected words into one 500 word article then yes, not negative SEO just crap article.
Does your site already have a theme? Is it (for example) a video site and now you want to conmpletely talk about a different topic? Not a good idea really.
If the words/phrases are linked in some way to your site, or your site is VERY general then go for low and high competition. Why not? Get the easy ones first and the hard ones later. Higher competition often bring better rewards in the end (that's why there is higher competition after all)
So if you have 100 words or phrases to optimize for, think of 33/34 articles of 600 words or more each. Each article optimized (loosely) for 3 words maximum.
Scritty