In their own words, from their site
TwitterHawk is a real time targeted marketing engine that will find people talking on twitter now by your chosen topic and location, allowing you to really hit your target mid conversation with ease.So twitterhawk will search tweets, based on your criteria, then send out appropriate responses to tweeters to provide meaningful information. Local stores could find customers looking for a particular product or service in the area. Many people consult their friends for recommendations, this would allow people to eliminate the middle man.
So how could this kill twitter? When I can't ask a friend where to buy rims without having ten Tire stores sending me spam/ads. Twitterhawk does have cost of 5 cent/tweet and spam-prevention policies. The problem will happen when me-too services popup that don't have the same good-hearted intentions. Then when I look for tires I'll get replies about how a fake degree will help me get 17" rims.
Hopefully twitter implements (or already has) safeties in place to prevent this. If not ill-intentioned copies of this service will drive me off twitter