Radak: Even though he was already liable, he died via the lookout. The lookout is liable for not warning him; he did not know what will be taken. The lookout was appointed to warn the nation, and they relied on him! "Veha'Elokim Inah l'Yado" (Shemos 21:13) ? one who killed b'Shogeg is liable, for he should have been careful. Chazal expounded "Ki Yipol ha'Nofel Mimenu" (Devarim 22:8) - he is called 'ha'Nofel' he was destined to fall (it was decreed from the Creation; even so, one must build a railing around his roof,] for good comes via virtuous people, and bad things comes via the liable (Shabbos 32a).

Malbim: If the lookout did not warn via the Shofar, even though the victim was taken due to his sin, I will demand (punish for) his blood from the lookout. He should have warned about the enemy, or aroused to Teshuvah. Via this, they would have been saved!