If Charles won and Altan died, Alvin could send men to finish off Altan's remaining followers. He might even be able to score a fortune from Altan's old subordinates.
Although Altan had lost his fief, he still held some influence on the steppes. His old subordinates surely possessed considerable wealth.
If he could wipe them all out, that wealth would naturally become Alvin's.
If Altan won and Charles died, Alvin could do Henry a favor.
Henry definitely held a grudge against Altan. If Alvin could present Henry with Altan's head, he might be able to earn Henry's friendship and a reward in return.
Alvin could use the opportunity to show his goodwill to Henry, perhaps even proposing some favorable conditions for himself. He could put in a good word or offer assistance when Henry needed it.
As for the fact that Altan would die in either of his imagined scenarios, Count Alvin didn't care.
