Ice damage shatters corpses, yes, hence you don't use it. And sanctuary aura on a merc does nothing for you but knockback undead, and council are demons not undead
Dreadfang has 33% chance to amp damage, which is by far the most reliable merc buff for the short duration of a trav kill since might won't turn on and reapers has cold damage and is weaker than dreadfang
its not too hard a decision here, they added an obviously vastly better weapon than the other options
does amp dmg work with zerk? i thought it did not