Why did the author have Mars Bar save Russell instead of Maniac?

I think that the author made Mars Bar save Russell because he wanted to show that Mars Bar wasn't really "bad", or cool. The author wanted the reader to know that Mars Bar could be kind. I think that if the author had Maniac save him, the book wouldn't have had as much of a good ending. The end of the book was definitely better when you saw that Mars Bar could think about someone other than himself. He appears to be mean and cold-hearted, but he is really caring and nice.