Darl McBride, chief executive of SCO Group Inc., says he sometimes carries a gun because his enemies are out to kill him. He checks into hotels under assumed names. An armed body guard protected him at Harvard Law School when he gave a speech last month.

Linus Torvalds, creator of the Linux operating system, calls SCO "the most despised company in technology."

The reason: SCO is claiming rights to the Linux open source software code that thousands of users and supporters say should have no owner. SCO brought a $50 billion suit against International Business Machines Corp. last year and last week turned on Linux users DaimlerChrysler AG and AutoZone Inc., suing for an injunction and unspecified damages.

"We are fighting the big battle," McBride said in a telephone interview from his office at SCO headquarters in Lindon.

McBride, 44, is pitting SCO against an industry it once helped develop. Less than two years ago SCO, formerly Caldera International Inc., was helping to form a standard version of Linux to compete with Microsoft Corp.'s Windows. Once McBride took the helm in June 2002, the company changed tack, hired attorney David Boies and began claiming that Linux users infringed on SCO's intellectual property.

Linux has attracted thousands of individuals and companies, some of whom see it as the only credible threat to Windows. Others use it because it's cheaper.

The software is now being used by companies ranging from DaimlerChrysler, the world's largest maker of luxury cars, to Lehman Brothers Inc, the fourth-largest U.S. securities firm by capital, to Google Inc., the world's most widely used Internet search engine. Lockheed Martin Corp., the world's largest defense contractor, also has servers that run on Linux as part of its computer network.

IBM pushes computers that run on the Linux operating system. Shipments of Linux-powered server computers, fast machines used to run Web sites, rose 53 percent in the fourth quarter, more than double the rate of Windows servers, market researcher IDC said.

McBride and SCO are more hated than Microsoft, the world's largest software maker, and its chairman, Bill Gates, according to some Linux backers. That's because SCO, once a backer of Linux, has turned around and attacked the essence of the system: its free source code.

"SCO are just complete hypocrites," said Jeremy Allison, co-author of Samba, an open-source software that runs a file and print service that SCO sells.

SCO says it owns the copyright to the Unix system and that parts of the Unix code have been copied into Linux. SCO is demanding payment from each user of Linux. Novell Inc. separately is disputing SCO's claim to Unix.