1. He'd suddenly forgotten every defensive spell he'd known.
2. He was curious what that curse does, so he tested in on Draco.
3. It was a just plot device.
Chat with our AI personalities
After the Sectumsempra incident in HP6.
Sectumsempra is a Dark Arts curse invented by Severus Snape (under his alias of "The Half-Blood Prince") for the use against his enemies, and it soon became one of his specialities. When uttered, its effect is the equivalent of an invisible sword, slashing the target repeatedly. The injuries appear to follow the movements of the caster's wand, and thus inflicts the most damage if cast with rapid waving movements. It works best on living targets and has no noticeable effect on inanimate objects or dead ones, such as Inferi, other than slashing them pointlessly. Sometimes the slashing cuts are continual, or it could be one cut.
In the book, Snape uses a non-verbal spell, so we don't know which spell he used, In the movie, the spell Snape uses is "Vipera Evanasca".
no
Nice ones I love harry styles he is me future husband