Thing is I think he did succeed at that part.
Back at the union meeting they were talking about how they have a "new system" that makes it much safer to use dangerous magic; that system is Sentarou.
Now that I think about it, his "bow down" is literally "bow down", but not revenge the uncle's father or sister. Like make the whole union who once morally rejected the uncle now can't live without Sentarou.
I think Sentarou has been in the union for about 15 years, because he looks like he's in his 30s. For all those years, what he's been doing is harvesting the souls for them, no sign of revenging or what. If so, he's led a miserable life imo. 10 years of being abused, abt 15 years of being exploited. And now the one who can replace him appears, and they immediately want to expedite the replacement.
This is just my thinking, it's not in vol 5 or 6.
Uncle did something to Sentarou so that when he uses the dog to kill someone that person's soul can somehow be used by another sorcerer to pay for magic. Which is insanely broken.
Wouldn't be surprised if it was already impossible for the union to go back to the days of members having to risk their own souls to cast magic.
You're really sharp on this!!! It will be in the next vol.
And tbh, I didn't expect someone to notice the sorcerers' talk at the meeting, because some of the dialogues are quite vague with the information provided at vol 3. Many Japanese readers say that whenever a new vol is out, they have to re-read from the beginning and realize the loose ends the author left in earlier vols.
The reason for their hate towards Sentarou will soon be revealed.