"MURDER! MURDER IN THE BATHROOM! MURDER!"
-Moaning Myrtle (HBP24)
A duel breaks out when Draco realizes that Harry has walked in on him crying and expressing his fears to Moaning Myrtle. Having been hoping for an opportunity to discover what the spell Sectumsempra does, Harry uses it on Malfoy. Harry regrets this and rushes to Malfoy when he sees that the spell cuts huge, bleeding gashes into its target. Myrtle is in hysterics, and water and blood cover the bathroom floor. Snape quickly arrives to heal Malfoy’s wounds with a countercurse, and orders Harry to remain where he is while Draco is taken to the Hospital Wing. (HBP24)
Before using this unfamiliar spell, the magic Harry used in duelling Malfoy here were spells typical of his usual strategy of neutralizing threats by immobilizing his enemies. -BB
When Harry hit him with Sectumsempra, Malfoy was about to use the Cruciatus Curse on Harry. Although Harry wasn't intending to do so, he matched this Unforgivable Curse with one almost as bad. -BB
Myrtle enjoys getting attention for her screams (HBP24), but perhaps there was some real fear in her shouts about "MURDER IN THE BATHROOM" since she was murdered in a bathroom. -BB
Snape, of course, knows the counter curse and is suspicious because he is the Half-Blood Prince.