Why would the killer kidnap instead of murder?

My MC needs to be kidnapped. The series of events leading up to the kidnapping is insanely complex, but the point is she needs to end up at a location so that she can be saved, and in turn save someone else. But what motivation would my antagonist have in kidnaping her, instead of just killing her out right, when he's proved more than capable of mindless murder in the past?

I'll take any suggestions.

Your killer has some weird ethic or protocol when it comes to killing women and the situation wasn't 'right'.

The killer wants to kill the rescuer more and is using her for bait.

She knows about something the killer wants.

The killer decides she's attractive and has err....other plans for her first.

That second one is perfect. Thankyou so much!

My serial killers are kidnapping my MC because she gets to close to arresting them and their plan is to kill her...until Derek her partner shows up.

The killer wants the world to know he/she has done it. The killer wants the ransom money. Killing was not the primary objective, the ransom money was but you know how it goes, he/she ended up killing him/her.

Maybe there could be a hired killer involved and he is a bit naive and clumsy, does not fully understand the assignment he was given to do and waters down the get rid of... to taking the person into hiding. Then the mix up continues.
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the victim or someone close to the victim posesses something that can incriminate the kidnapper, so when he kills the victim he will never get it back.

Maybe the villian wants the victum to experience the same pain they went through, so in a way it's like getting revenge by torture.

There's a brilliant kidnapping twist in Julie Zeh's Dark Matter that you could possibly take from, simply that the kidnapper misuderstands/mishears the instructions they've been given. Eg a message saying 'take them' as in take them out is interpretated by the kidnapper as take them prisoner. This would be very easy if you had a language barrier, so the kidnapper and their boss do not have the same 1st language.