(SPOILER) Did Harkon really intend to...

[CaptMorgan72]

kill Serana? He only needed her blood and he could get that without killing her. I use her blood to create the arrows to darken the sun and she is none the worse for it. Or did Harkin's original plan to permanently darken the sun require all of her blood? I know he saw the dragonborn vampire as a threat to his throne and intended to kill him/her, but what about his daughter?

 

[Octavia]

Well considering Harkon will pretty much do anything to achieve his goal, and not to mention he doesn't even care about her anymore, I'd say he probably would've just killed her in cold blood. Yet another reason for me to hate his sorry ass, I made sure his death was slow and painful.

And I'd assume using her blood on the bow itself would create a much more powerful effect, and would require more blood than what was used on the arrows.

 

[UndeadDollface]

Honestly I found the details of Harkon's motivations kind of murky and contradicting. I get that he had become so obsessed with his goal of darkening the sun to the point he was no longer considering any of the repercussions or costs, but his reason for needing to kill Serana and the Dragonborn vampire seemed to keep changing (e.g. out of necessity, for perceived treason, or a potential threat). Maybe it was just his insanity or maybe I need to play it again, but it felt like the writers were running out of steam towards the end and pretty much said: 'Look, you know you're going to have to fight him eventually so lets just get to it.'

 

[CaptMorgan72]

Actually I think it is pretty simple. He didn't care about his family anymore, only about achieving absolute power. Permanently darkening the sun would require all the blood of a pure blood and since his wife was in hiding, that meant his daughter. The bow would have to be submerged in the blood of a pure blood and under go a ritual that would bring to fruition the prophesy. The dragonborn proved to be a threat to Harkon's throne and would have to be eliminated. That's it pure and simple.