My main domain is domain.com (for example), but i want to buy:domain-dragon.com  domain-killer.com  and on and on and redirect (301) to my main domain - domain.com, is this considered as a violation of SEO rules. Is it bad for business?
Redirecting different domains to your main site
seo;domains;301 redirect
It is not bad for business and it is not bad SEO if all you do is a 301 redirect when you hit any of the other domains. Google even considers that a good thing because you are essentially telling them that those other domains are also just owned by you.Tons of major sites do this already like American Airlines, americanairlines.com redirects to their main site (aa.com), and bn.com redirects to Barnes and Nobles main website.  Some sites even own common misspellings and do 301 redirects from those domains just to make sure if you mistype the domain you still find them.Where you can run into issues is when you create content at domain-dragon.com that is a duplicate of a page on domain.com and add links to it that go to the duplicate content page on domain.com.One last thing to do is make sure your 301 is setup properly by using a tool like http://www.internetofficer.com/seo-tool/redirect-check/ or fiddler to confirm your redirect is a 301 and has all of the proper header info it needs.