The option was there in beta.
And many players killed him off. Afterwards the majority of players that killed him off, submitted complaints to bring him back because they missed having a healing companion.
So, BW removed the ability to kill off companions.
Here is what they could have done. If you killed a companion off perhaps they could make it that replacement with similar abilities immediately turns up. ((i.e. You kill Malavai for his treachery, one of the Emperor's servants joins you as a replacement healer). It is almost like in Suikoden 2, if you let Anubis get killed, that golden retriever dude joins you instead.
I thought a good one would be ((Mako spoiler))