Well, the party is on a mission that is beset by danger already from the Vitlings, Winter Wolves & basically everything, right? So getting two foes to go at each other may be a good thing for the groups survival & the success of their mission. This kid says he wants to serve his people but when faced by bared fangs his natural instincts have so far kept him alive.
Without taking into account morality, I would point to persuading the kid to come with us as a guide to stay alive and I'd let the two groups fall out with each other. The undead are not part of the situation, they are a peripheral concern.
As for the party presented, I'd rationalize it as a means to improve the odds of success + doing good locally to save a life. The bigger picture is overwhelming & the party has neither the power to change it nor enough knowledge to understand the full implications. I say act locally & let nature take its course in the grand scheme.