Biofuel is actually a transparent liquid. This thing has an extreme affinity with anything that has a wood element in it and this man that came to kill him actually has an elf bloodline in him.
The trio might have difficulty identifying them, but Sam could do so with just a sniff.
This is one of the perks he got from the vampire body he obtained.
The blood spilled from the intruder was everywhere and with a sniff, Sam could guess what he is.
He is the offspring of an elf and a human. But the human bloodline dominated the elf bloodline which made him unable to access the wood element with his body.
But there is a faint trace of the wood element in his blood which is due to the elf's bloodline.
He wanted to test this bio-fuel on a live subject and since the intruder is here, he is doing Sam a favor by testing his new potential weapon in his arsenal.
