Does the text for the Bloodletter card apply to all damage issued during combat? Or does it apply to both attacking and defending units?

All damage to units, no matter who controls them. Not just in combat.

so your comment about "not only in combat" makes me question something else. Does this also apply to tactic or actions? Does it also apply to any damage even if bloodletter is not in play?

thanks.

It applies to all damage to units, not just combat damage, since the card says "double all damage assigned to units", not "double all combat damage to units". Naturally the effect only exists while the card is in play though.