Why were An Jung-geun and Yun Bong-gil sentenced to death even though they killed the enemy?

When listening to the stories of An Jung-geun and Yun Bong-gil, they shot or threw bombs at filthy individuals like Ito Hirobumi or General Shirakawa—people who absolutely should not have been left alive. Why were they still sentenced to death?
It was said that if they had let that guy Ito Hirobumi live, China and Russia would have also been in danger...
They killed enemies who deserved to die on a battlefield, so why were they sentenced to death regardless?