Does the speaker think that the war is worthwhile?
I would say that the speaker in this poem does not think that the war is really worthwhile. On the other hand, he does not really regret coming to fight either.
To him, the war is pointless. He doesn't hate the Germans and he does not love the British. He says that the war will not have any effect on his "country" or his "countrymen." So there is really no point in the war, in his opinion.
But he doesn't regret it either. He seems to me to be saying that his life was pretty pointless before the war and that he only came to war for a "lonely impulse of delight." He seems to think that life after the war would be "a waste of breath" too. So he is content enough with the idea that he will die fighting this pointless war.