Because he wanted to be relieved of his mission. Dad refused, and so he poisoned him.
8 seconds sound clip from the Strife for Mastery (1978) movie soundboard.
You can hear this line at 00:39:38.383 in the DVD version of the movie.
Quote context
[...]
- It was my own brother who killed him.
- But he had no reason to do it.
- Oh? But why should he want to betray you though?
- Because he wanted to be relieved of his mission. Dad refused, and so he poisoned him.
- Because he'll be at Star Cliffs by now.
- Tell us.
- We tell the priest what your brother did, and convince him.
- But still, we have no proof.
[...]