In riftbound. If you debug a unit to 0 or even negative might. Does it die?

No — dropping a unit's Might to 0 or below doesn't kill it by itself. A unit only dies when it has a non-zero amount of damage marked on it that's equal to or greater than its current Might (Rule 143.2.a / 142.4.b); Might reduction is not damage.

So a 0-Might unit with no damage on it just sits there alive (and useless — negative Might is treated as 0 whenever spells, abilities, or combat damage sums reference it). But if that same unit already has 1 or more damage marked, it now has lethal damage and will be killed in the next cleanup. Note the check is against its current Might, so a unit debuffed to 3 with 3 damage already on it dies too.