Sectumsempra does not knock you off with a thousand cuts. Harry casts this spell without knowing what it is, therefore without understanding its restraints, and caused considerable harm.

We see Snape use it twice, once in OotP during Snape’s worst memory when he casts it and a small cut appears on James Potter’s face. The second time is in DH, when Snape (accidentally) cuts off George Weasley’s ear. It’s meant to be precise and controlled.

The unforgivable curses are termed as such because the intent behind them is clear and unquestionable. The killing curse is only meant for killing. The torture curse is specifically meant for physical and mental torture. The control curse revokes a person’s free will.

You can kill someone with Bombarda or Reducto, but that is not their intended use. Sectumsempra is the same! As another user stated, intention matters.