Having followers with powerful strength is indeed gratifying, but Liszt values the character of his followers even more.
Yevich, who has repeatedly turned against landlords and now is willing to serve the invaders for a bit of ransom, certainly has a very low character.
On Earth, he would be akin to the turncoat Lü Bu—of course, Lü Bu was the unparalleled warrior of his era, while Yevich is not even particularly strong among Sword Saints, so the difference in strength between them in their respective eras is immense. At most, Yevich has only turned against landlords, not committed patricide for glory.
It is not uncommon for landlords and followers to fall out due to personal disputes.
If it's just about character, accepting his allegiance is not a big deal. Even the most despicable person, once under his command, would have to obediently accept the allocation, or else Liszt's Sky-Cleaving Greatsword would show no mercy. Be it dragon or snake, they must coil within his territory.
