It would be politically unacceptable for them not to condemn the killing, even if they approve.
If Pack B is subordinate to Pack A, but Pack B goes around killing Pack A members, they're not really subordinate; they're more or less at war. Since Pack B ended up becoming subordinate in the first place, they clearly saw some reason for this: they weren't strong enough to kill Pack A, or there was some benefit to them falling in line. Unless things have changed, Pack B values that relationship (which might be a matter of life and death for all of them) more than it values supporting the murderer. It's a cold analysis, but a logical one.
Conversely if they don't condemn the murderer, Pack A has no choice but to assume that they intend to kill more of them. They'll muster their response, and no matter what happens, that (presumably beneficial) relationship is pretty much gone. If Pack B thinks they can handle the fallout without Pack A, then maybe they won't condemn this, and will escalate the situation: now you have a rebellion. But are they confident they can win?
This is assuming that Pack A and Pack B's relationship is basically feudal, that one is subordinate to the other, but the logic works out pretty similarly if they're in a coalition of equals. The fact of political life is that there are people we dislike, maybe even enough to kill, that we have to get along with for purely practical reasons.