Everyone knew that Lavender Brown was dead; everyone who saw her go down, that is. Fenrir Greyback bit her throat out just before he himself was killed at the final battle of the Second Wizarding War, the Battle of Hogwarts.
Luckily, Seamus Finnegan did not see this, as he had wand-to-wand combat of his own at the time some distance away. For him, the battle ended when he found his worst fear: her bloody body on the flagstones. Anyone else would have realised she was beyond hope and returned to the fray, perhaps with renewed determination, anger, or even blind rage among those who loved her.
The blind rage that probably would have gotten him killed did not enter Seamus' mind. The distraction from his surroundings that should have made him an easy target also failed to get him killed, as luckily the tide of battle swept away from Seamus and Lavender. Luck played a large part that night, oddly balancing good and evil in the haphazard way that only Luck can accomplish.
For instance, Lavender was both lucky and unlucky to have been attacked by Greyback, the most feared werewolf in Europe. She ought to have been killed almost instantly by the damage to her throat, and was lucky to be alive. She was unlucky in that what saved her was the werewolf's Curse. Lycanthropy is not a magical disease with a cure, nor is it a heritable condition: it is terrible curse, spread by attack only. Part of the effectiveness of the Curse is that it allows its victims to suffer great damage (and pain) without dying.
Lavender was also lucky to have been found by Seamus. As lycanthropy is not at all a healing spell, it could only do so much. The artery and vein to her brain on the left side of her neck had not been severed, but she had gone into shock while the Curse had acted as a kind of magical tourniquet on the right side. Also, her torn trachea was slowly seeping blood into her lungs, and her heart was barely beating as Seamus knelt beside her.
Seamus simply did not accept that Lavender was dead. It was inconceivable.
