The speaker asks to be buried in a spring garden, a place of new growth, and then asks for something stranger: to be emptied of "use and art" and filled with plague. It's a disturbing request. The speaker wants to give up everything that makes a person useful or creative and become a carrier of death instead.
The second stanza gives the reason, or at least a sour kind of one. The news says people must go to war, and the speaker's answer is to volunteer in the most grim way imaginable. The final lines, "so I may prove I did / what I could," sound weary and a little desperate. The speaker doesn't seem to believe the sacrifice will fix anything. It's more about needing to feel they did something.
The mood is bleak, numb and quietly ironic. The contrast between the lively spring garden and the plague makes the wish feel even more unsettling, as if despair has taken over a place meant for renewal.