Lucien has no need to completely drain anyone for blood.
If you've read any of the Gone From Daylight series, those vampires prey on criminals. And there are some who freely give blood just as Eli did here.
It was a problem in all major cities back in the day. In NYC, it was put on Barren Island to be turned to fertilizer. You didn't want to be on the wrong side of the wind of that place.
The cost of Lucien's freedom is unveiled, and it's high. The four family bloodlines are non-negotiable deaths and likely Kylie.
Lucien's love for Eli is both deep and true. He is willing to endure rather than hurt Eli.
There are so many things which can go wrong.
It was all easier with the campus shutdown. With the Vespers back, Eli has to perform a version of himself which existed before he got the blood. It's interesting Lucien views her as the greatest threat. She examines things like Eli does.
I am reminded of the joke about the man at an evangelical meeting being caught up in the frenzy and stood up to confess all his sins. As he did so they shouted out, "Tell it all brother. Tell it all."
He hept going and suddenly there was absolute silence. Then the preacher said, "Brother, I don't believe I would've told that one."