The mother and father of the killer are failures. They are responsible for the murder of children. After all, parents have a duty to care for the health of their children and talk to experts when teenagers are pyscho and suicidal.
Normally I would agree, except the 18-year-old is not a kid. It's one thing if there is clear evidence the parents abusively raised their child and contributed to their kid to have mental issues. But to assume the parents are fully responsible for their young, "adult" son to demonstrate evil acts without us knowing the entire story cannot be justified. If he went nuts due to being high on drugs, or his girlfriend just broke up with him, can we blame his parents for his crime?