A beaver killer does not truly kill, but cripples the erotic drive of the victim. It's a cock block from the source, the girl's perspective. Chopping down the self esteem of the male like a nice big oak tree, the beaver killer is the highest form of sexual denial.
"Did she gnaw at your tree? Did she open the dam?"
"Nahh, she said I was a beaver killer and swam away gracefully amongst the mermaids."