If he was sentenced to death, then he must have been convicted for murder. It's the only crime for which death is a permissible punishment. Basically true, at least in terms of state offenses. I think on the federal side, there's treason and perhaps a few others. A few years back I remember being amused to discover that in this day of detailed statutory regulations and complex sentencing guidelines, the federal criminal code still defined the offense of "piracy" in language something like this: "Whosever cruises against the vessels of the United States is a pirate", with death being the specified maximum sentence.