I was only a little bit saddened when I got the quest as vampire to kill the dawnguard leaders without any second option. I had hoped that after getting the bow that I consider to be too great an evil power to unleash on tamriel should be handed over to the Dawnguard if possible. I'd love a little conversation between me and isram going something like "Sup, I killed the leader of the volkihar clan, took his power, and got Auriels Bow. I don't like these feral vampires attacking any more than you do, here take the bow to keep skyrim safe." Isram: "I would have never expected this! fine I'll take the bow and let you leave but don't expect a warm welcome if you return, but I will speak of your honor. This should help us put a damper on the vampire attacks."

Is the kill Dawnguard leaders thing the last quest from the vampires? or is it just one of those quests where i'll go killing people for the vampires or whatever over n over forever now?

It's a side quest. There will be more. But you will remain enemies of the DG for eternity.

Isran and peace with vampires does not mix. Plus he considers serana an it than her and lets not forget you spent majority of the quest line killing dawnguard plus they are vampire hunters why would they make peace with the very thing they hunt?

I think this would have been a great alternative for players to have. I also have a good deal of resentment for Isran demanding I get a cure after his initial quests -while at the same time Serana is wandering around Fort Dawnguard because Isran let her in as a Vampire on my say so...

In case it's not obvious I would have liked a 'benevolent' Vampire option in the tradition of Janus Hassildor. A 'good' Vampire would have been a powerful ally for a bunch of Vampire hunters. Too bad Isran is a zealot.

With the kill the Dawnguard Leaders quest, it's randomly generated after you finish the main Vampire quests. In my game I Pacified each of the spell sellers, purchased all of the new spells, killed the leaders for the update and then resurrected them all with console commands (someone has to still be there to look after their dogs). Completely idiotic and completely messed up storywise, I know, but I want them to remain in my game even if I never visit them again. I also never kill the named Dawnguard members you meet on the road. Yay, for being a Vampire and being so powerful with Illusion.