I wouldn't characterize it as altruism as much as a sense of duty. The Tremere are very into duty. They also see themselves in some ways as an army. The mindset that they're in army would probably lead them to ask questions such as "is it our duty to wipe out [faction]?". On some level they believe that being an army is a advantageous perhaps even necessary thing to be. If they're convinced of the value of being an army they're going to look to find the factions that need to be fought (which must logically exist otherwise they'd be wrong about the wisdom of being an army, and the Tremre know that they're not wrong).
It's not about specific spirits, it's about the relationship between humans and spirits. It's the opinion of the house that gods always screw over the humans (the christian god may be an exception because he had the wisdom to sacrifice himself for the benefit of man). If the Tremere believe that that the Order of Hermes, aka us, is sacrificing humans to spirits they'd be pissed. "My organization doesn't do that, that's what fools do." Perhaps that's motivation enough.
I agree. This is a very understandable, human motivation and it's a lot more believable, IMO, than some sort of Realpolitik Grand Strategy.
Of course, the Tremere (and others) took the opportunity of the Schism War to do some political manoeuvring. And political ambitions certainly influenced the way that the Schism War escalated and was eventually resolved.
But this comes back to one of my earlier points. In Transylvania, the Tremere are surrounded by people who have magical or religious relationships with spirits, including bloody relationships. The shamanic people from the steppes, including the still-pagan Magyars, are the most obvious, as well as the pagan Slavs and whatever local Dacians are left in Transylvania itself, which was abandoned by the Romans prior to the adoption of official Christianity. If the Tremere are bothered by bloody pagan rites, they should be in constant war with their neighbors rather than worrying about the Bretons.
Sure, if that is the case the Tremere are probably bothering their geographic neighbours too.
However, the point is not so much about the fact that some Bretons might have been sacrificed. The point is that magi of the Order of Hermes were supposedly doing the sacrificing. It's partly about purging the Order of "Fools That Are Doing It Wrong".
I would add in a bit of an inferiority complex, so they would feel they have something to prove. I think Germany in WWI is a fairly good example of this, they have a strong sense of house loyalty and pride and yet as individuals they really are not as exceptional as many others in the martial realm.
Ah but those are not people who are in the Order of Hermies. Given the general power difference it and politics it is not at all surprising that mages would discount such people. That would be a much more altruistic act, than fighting with in the order.
In the saga I am playing now, it is a little bit special...
My Flambeau, Fusta of Ramius School has killed a Diedne magus hidden in a cave, who was in reality an ancient Diedne covenant... but she kept the books of the ancient covenant (a gifted Mercere has repared many books so that they are readable and a ghost of a scribe from the schism war taught her and other from the covenant the language old gaelic, in which is written most of the library).
So in one way, she would kill any Diedne magus she would meet, but in another hand, she likes to keep books in old gaelic, with some who are forbidden because written by Diedne magi...