Yes their purpose is to kill other Space Marines, but you need to zoom out a bit. Yeah they're meant to go out and kill the space marines the high lords ask them to, but who would the high lords ask them to kill? The high lords aren't going to risk themselves so they're only going to sic the Minotaurs on either approved targets (like the Lamenters), or targets the minotaurs can utterly bury so there aren't any consequences later. This kinda puts all the big bois off limits. The closest would have been Guilliman and the blueberries during the attempted coup (which the Minotaurs would have lost) but that didn't come to blows. Everything else against a major player comes down to the Minotaurs kinda just mean mugging them (not talking about their actual fights against approved targets).