The hacker said their prime motivation for the backdoor was to build a botnet.
A lone hacker who duped hundreds of users into downloading a version of Linux with a backdoor installed has revealed how it was done.

News broke on Saturday that the website of Linux Mint, said to be the third most-popular Linux operating system distribution, had been hacked, and was tricking users all day by serving up downloads that contained a maliciously-placed “backdoor.”

Source: ZDNet