a couple things:
First off, to KebreI: If you have never had just one family member worth a sh--, then you probably won't understand this. Schala raised Janus, not Queen Zeal. Schala was the warm one who shared in Janus' successes and failures. She was his shoulder to cry on, his handkerchief to wipe his nose and more than likely the only person Janus really got to play with due to his royal upbringing, before he got stuffed into the Middle Ages. To Janus, the boy, Schala is Sister/Mother/Goddess/Friend/etc. To Magus, the man, she is the one thing left pure in his life, when all else has ever been taken from him and ripped to shreds, he had the memory of his Schala to keep him going. Her support was enough that he never forgot it in all of his years of being made the monster by the mystics.
as for the growing argument about why Magus joined the group, helped them bring Crono back, I have a few theories based on Magus' Personality. First off, Magus is aloof, so he doesn't always say the things he wants to say. He comes off as cold, unfeeling and callous, yet he isn't those things. He feels remorse for his actions, yet is too prideful to apologize. He calls Crono and Co. weak because he sees them doing things that he considers to be weak, yet is confused when they succeed and he fails.
If at all anything, curiousity about the strength found in such weakness would make Magus aspire to view firsthand how they worked. A conscience would be another reason. Perhaps Magus felt the tiniest of pangs of remorse for all the evil he had done in his pursuit of destroying Lavos. Perhaps he felt sorry for ruining Glenn's life and Cyrus' and finally Crono's and wished to make it up with his death. He offers no resistance when you choose to kill him, except for the mandatory 'fight for your life' thing.
Magus doesn't fear death, which means that he has nothing lose. He has nothing to lose by Glenn killing him at the cape there. He has nothing to lose from joining up with them, yet he stands to learn so much more about how they work together and make strengths of their weaknesses; he has a chance to settle up with Lavos with a good group of strong fighters, and he stands a chance to redeem himself with the information that Crono can be revived. I don't think Magus sits too well on ruining the lives of three great swordsmen. There has to be a sort of respect there, even with the insults.
Really, just things to consider. Magus never really was the bad guy, he was just never really the good guy, either. He traveled whatever roads lead to his ultimate goal: the destruction of Lavos. I'm sure that to a haunted man such as him, anything other than revenge would have been an unexpected bonus.
