5th Ed. Codex: Blood Angels:
Thus rejected, Horus flew into a rage and attacked. Even at the peak of his powers Sanguinius could not have hoped to prevail against the monster Horus had become, and the Primarch was weary and wounded from his travails on Terra.
2nd Ed. Codex: Angels of Death:
While he was still alone he chanced upon Horus and, although he knew it would almost certainly spell his doom, he immediately attacked the Warmaster. But Sanguinius was no match for Horus at the height of his daemon-provided power, and was slain by the Warmaster with contemptuous ease.