Buff is always positive, negative is called in this game “ailment”.
Please see the whole topic on that case:
https://forum.smallgiantgames.com/t/status-effects-buffs-and-ailments-and-stacks-dead-topic-see-link-in-post-190/110620
And as Kalgaath said, skadi works fine as she is casting stacts on enemies not on herself. So if all heroes were killed, there have no stacks. Each wave is a new set on enemies.
But you will probably say that I am another one with no connection to the brain 