I killed Magus in Chrono Trigger. Did I miss the opportunity to have him in my party?
I've been replaying Chrono Trigger on the DS and have gotten to the point after the Ocean Palace where you fight Magus for the second time.
I think I killed him...
Have I missed the opportunity to have him in my party?
Unfortunately yes. When you can choose whether to fight Magus or not, that decides whether he can join you or not.
After defeating the Blackbird [...] if Frog [or the Party] refuse [to fight Magus], Magus will join the party.