懒得翻译了，自行观摩一下
1. He (and perhaps his family) have known about them for many years and choose to feed them out of some sense that they need protection, looking after, etc. If this is the case, they are probably feed animals almost exclusively, but he (the old man) would appear to not be above killing people for them.
2. He could simply be a sociopath, and like any serial killer, killed her because he likes to kill people. Perhaps he's done this many times before, and dumping the bodies at the mouth of the cave, where he knows they'll be eaten, could be part of his enjoyment.
3. He may feel that if a lot of people were to find out about them, their survival would be threatened. As such, he takes steps to protect them. Keeping hikers, campers, hunters, stranded motorists (he does drive a tow truck), etc, away helps, but sometimes he feels the need to go as far as killing someone who has come to know too much.
4. He might feel that he needs to feed them because if their natural food supply were to run too low, the Crawlers might start venturing out greater distances and begin actively hunting people. If this is the case, surely his preferred choice of food for them would be deer, elk, etc, over people, but he may also feel a combined need to protect them. As such, he killed her to keep her from telling others what she knew.
http://www.imdb.com/title/tt1073105/faq#.2.1.3
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