Pretend to accept, then backstab him
From Create Your Own Story
If you pretend to accept his offer, maybe you can get close enough to stake him. You touch your clothes where the stake makes a slight bulge. You hope he doesn't notice it.
“Okay. I’ll become a vampire. What do I have to do?”
“Nozzing much. Just let me bite you.”
“That’s it? I don’t have to drink your blood or anything?”
“No. I vill put somezing in your veins vhen I bite you. It’s simple.”
“Will it hurt?”
“A little, but I can make it… qvite pleasant.”
“Will I be subservient to you or anything?”
“Partially, but far less if you submit villingly. You vill need to take your crucifix off, of course.”
Somewhat hesitantly, you lift the cross over your neck. You put it down on one of the pews. You walk over to Walfreid, who stands to meet you.
He comes up close and puts a hand around your waist. His eyes bore into yours and unconsciously you relax. He kisses you on the mouth. While this is certainly pleasant, you have a job to do. You get your stake out. He moves to your neck. You make as if to embrace him, but plunge the stake into his back. It goes in surprisingly easily.
Walfreid looks at you in shock as he stumbles to the ground. "It seems I underestimated you. Vhat a shame. You vould have made a damn good vampire." His eyes close and his face pales. He crumbles into dust.
You stand there, breathing hard. It's hard to believe you actually did it. You retrieve your crucifix and hang it around your neck again. On an afterthought, you scoop up the dust that was Walfried, and bring it outside with you. You scatter it on the wind.
It's fully dark now. A howl sounds somewhere in the woods. It's a long walk down, and there are other unknowns in that woods. The castle should be safe now that you've killed its inhabitants.