He was shooting GH into his son because he wasn't big enough at 7 years old, they're saying in the news.
Fox News just made a report, stating that authorities believe Benoit used a wrestling choke hold on his son. Also mentioned was that Benoit and his wife were fighting, with the Mrs. demanding that Benoit spend more time at home with their son, who has some sort of disability.
That's somewhat strange, considering that Mrs. Benoit (a former wrestling manager, "Woman") knows the long road schedule that wrestlers have.