In the movie salems lot why did barlow never kill mark?

Mark managed to escape when the vampire Barlow killed his parents, thanks to the priest. After that he continued to evade capture but when he was eventually taken prisoner and tied to a chair in the crumbling mansion to await being bitten by Barlow after sunset, he was able to get out of the ropes because he had studied magic and escapology. After that he joined Ben in the destruction of the vampires and the house and escaped with him, to continue their crusade as vampire hunters.