Sectumsempra is not technically a killing curse. Avada Kedavra instantly kills the victim and there is no counter-curse or way to revive someone after they have been killed. Sectumsempra is more of an assault curse. If it were more widely known it might very well be illegal but it was made up by Severus Snape which meant it hadn't been around for a long time and he didn't widely publish it.