Why wouldn't you attack Serana on sight?
1. Other Vampires are searching for something - turns out it's her - that makes her important - you'd want to know why.
2. As soon as you see her you notice something she has - An Elderscroll - so you'd want to know how and why she has it.
3. As AegisRunestone says - you don't know she's a Vampire until you talk to her (at least the character doesn't).
As for "walking her home" instead of killing/leaving her - well, in doing so (and making it out alive), you now know the exact location of the Vampire stronghold, and also have a rough idea how many Vampires there are (at a minimum). That, I would think is pretty valuable information. You also learn that Harkon is more interested in the scroll than his own daughter - so there's information in that alone showing that there is something much bigger occupying Harkon's mind.
Charging in hacking and slashing doesn't always get the best results. Think about what would have happened is you had somehow forced Serana back to Fort Dawnguard in the first place.
You have a run down fort - Isran, Durok Celann and the farm boy. Maybe one other recruit I can't recall if anyone else had signed up at that point. Now, you take her there and Isran either manages to get her to talk or kills her. If she talks, the only thing she'll know about is where her home was - nothing about who or what is there - remember she doesn't even know who might be there when you first go to Castle Volkihar (Harkon or Valerica), nevermind what sort of entourage they might have.
If he kills her - right, one dead Vampire, and an Elderscroll they have no idea what to do with. A scroll that Harkon's looking for. Knowing the way rumours travel in Skyrim - it wouldn't be long before Harkon learns of the discovery of the scroll, and the potential death of his daughter. He's going to want that scroll and would likely assault the fort as soon as possible in order to claim it - no time for the Dawnguard to build up forces or learn of the attack. It'd be a slaughter, an outright one if Harkon goes himself and uses his Vampire Lord form as well as bring everyone with him (his court members, the hounds, maybe some other lower level vampires).
Especially since Isran at the start is too stubborn to admit he needs serious help with the effort, so at the most he'd have a few extra - untrained - footsoldiers at his disposal as well as Durok and Celann. A few crossbows and Isran's little lightshow spell aren't going to help if 10+ fairly powerful Vampires suddenly come kicking the door down and letting their pets out to play.
So instead, you take her home, learn the odds and where everyone stands a little better and give Isran enough information to start realizing that he needs proper backup on this matter - enter the search for a more experienced team and also building up the rookies and equipment. Harkon in that time carries on as was and makes no real efforts to do anything other than sending out a few scouting parties to see what is going on, and grabbing a Moth Priest to read the one scroll he has. Which doesn't help him, as I doubt he'd ever figure out how to obtain the one Valerica has (as I doubt Serana would offer any information to him - considering she didn't think of her Mother hiding in the Castle either) or where to start looking for the third one - seriously, all that time and not once did he think to get someone asking at the college of Winterhold about Elderscrolls?
At least he wouldn't think of this for a long time, if at all - which would give the Dawnguard plenty of time to build up and plan their attack with the proper forces, equipment and strategy.
Better than running in blind and ending up on evening menu.