My main domain is widget.example (for example), but I want to buy:
widget-dragon.example
widget-killer.example
and on and on
and redirect (301) to my main domain - widget.example, is this considered as a violation of SEO rules. Is it bad for business?
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 widget-dragon.example that is a duplicate of a page on widget.example and add links to it that go to the duplicate content page on widget.example.
One last thing to do is make sure your 301 is setup properly by using a tool to confirm your redirect is a 301 and has all of the proper header info it needs.
Are you a huge company with unlimited resources, like American Airlines?
Are you a company with a bland Acme-style name that's easily mistaken for a hundred other companies?
If no, buying a bunch of domains just to redirect them to your site sounds excessive and like a waste of money. Put those resources into creating great content.
