I think that the way in which Twain depicts Brown ends up elevating Bixby's role as teacher or mentor. The experiences that Twain has with Brown represents some of the worst aspects of instruction. Twain's experiences with Brown border on savage cruelty. Twain's introduction of Brown, as a character, is one that reflects a different reality than Twain's relationship with Bixby:
He was a middle-aged, long, slim, bony, smooth-shaven, horse-faced, ignorant, stingy, malicious, snarling, fault hunting, mote-magnifying tyrant. I early got the habit of coming on watch with dread at my heart. No matter how good a time I might have been having with the off-watch below, and no matter how high my spirits might be when I started aloft, my soul became lead in my body the moment I approached the pilot-house.
Twain's description reflects an alienation and estrangement that he feels with Brown as his mentor. This is not evident in the relationship he holds with Bixby, reflective of the manner in which Twain learned from him. It is also indicative of the lack of real learning that took place with Brown.
In Twain's experiences with Brown, there is an emphasis about the condition of learning. Whereas with Bixby, there was an embrace of the learning, instruction process, and love of the Mississippi, this was not as evident with the way in which Brown carried out his "instruction:
Brown was always watching for a pretext to find fault; and if he could find no plausible pretext, he would invent one. He would scold you for shaving a shore, and for not shaving it; for hugging a bar, and for not hugging it; for 'pulling down' when not invited, and for not pulling down when not invited; for firing up without orders, and for waiting for orders. In a word, it was his invariable rule to find fault with everything you did; and another invariable rule of his was to throw all his remarks (to you) into the form of an insult.
The idea of instruction as a form of entrapment is different than the mentoring and sense of intrinsic love that Bixby nurtured. Twain's experiences with the pilot Brown make him display a reverence for Bixby. The respect that Bixby shows towards his cub is fundamentally different than what Brown displays towards him. When Twain talks about the number of times he "murdered" Brown in his own mind, the level of anger that Brown has inspired in him is matched by the esteem with which Bixby is viewed. Bixby represents the ideal of teaching in comparison with Brown, who ends up embodying the very worst in instructional aims.