I'm pretty sure a lot of people shared your opinion on the pardon even back then. I do. I think it would have been okay to grant some lenience to them because of noble motifs, like not handing out a life sentence instead of a death sentence or 20 years instead of life for his accomplices. On the other hand – WWII started a wild time in the world with revolutions, armed conflict (look up the strike culture in the US in the early 20^th century!), civil wars and full scale wars all over the place where the life of a human being was not worth a lot. Probably the morals regarding blowing up strike breakers of today and of 1908 are more alike than these of 1908 and 1917.