Charles Whitman was a brilliant, yet troubled young man, having murdered both his wife and mother before committing suicide. Post-mortem autopsy of his brain revealed a glioblastoma multiforme tumor the size of a walnut, erupting from beneath the thalamus, impacting the hypothalamus, extending into the temporal lobe and compressing the amygdaloid nucleus. This could explain why he had the mental issues that led him to kill. However, a toxicology report could have helped police and his family better understand his conditions and motive, since they do not know whether he was on drugs.
Charles Whitman was a former Marine who killed civilians at the University of Texas before being captured by the local police. He also murdered his mother, reasoning that he spared her from the mental torment of separating from her abusive husband. However, in his notes he mentioned that he wished post-mortem to have an autopsy performed to check his brain for mental illness. An autopsy revealed a brain tumor, which may have affected his personality, but a toxicology report would have confirmed whether the tumor was caused by cancer or a potential chemical.
While I would not ever blame his actions on any substance or drugs in his body, it would be interesting to see if and what he was on when he committed those horrible murders. Drugs can change someone but I don't know that drugs can turn a decent human being into a mass murderer.
He's dead. It doesn't matter. Toxicology will not change that. It was done and there is nothing that can change it. All a toxicology would show is possibly why. I don't see any reason to do that when there is no punishment that could be meted out. Even if there was it seems worthless to do.