Can a blood harvester overkill themselves with frenzy? Example: I have one unit who rolls five dice and gets five helmets. Can I kill all the opposing units or do five damage, or just one kill/damage since that kills me?

I think they can kill themselves.

Read "Frenzy" on page 5 of the reference book. It says any number of Heroic results can be committed to cause 1 damage each, regardless of the number of units remaining in the unit. Then you remove one unit for each damage you caused with the ability.

So yes, Blood Harvesters can overkill themselves.

Overlooked that one. Good to know!

Don't feel bad about missing it. I sent in a Rules Question about Runic Barrier to FFG and saw "Rune Blade" but no "Runic Barrier" in the reference book. They emailed back and directed me to the reference book and sure enough, it was there. I had just skipped it the first time.

So, can a Blood Harvester kill a Roc Warrior unit with a single helmet?

If the Roc has only one life left, why not? Remember that Frenzy deals 1 damage / die committed; a Roc Warrior at full health requires four damages to be killed.