Why does W. B. Yeats use an ambivalent tone and attitude in his treatment of the political uprising in his poem "Easter, 1916"?

"Easter, 1916" is a poem in which Yeats expresses his feelings about the Irish uprising against British rule led by reactionaries he knew personally. The poet or speaker is trying to come to terms with both the revolutionaries' violence and subsequent executions by the British government. The poem's speaker struggles between his distaste for violence in general and his admiration for the ultimate sacrifice made by these rebels. Several lines in the poem are ambiguous in nature and serve to express these conflicted feelings. The first is "A terrible beauty is born." This line is mentioned at the end of the first, second, and final stanza. This line is preceded in each case with a mention of change

All changed, changed utterly

The first stanza describes the ordinary lives of the rebels and the poet's casual encounters with them before the uprising. He would often pass them with "a nod of the head" or an exchange of "polite meaningless words." Now, all this is changed. In what way and for better or worse, the poet does not say. He just knows a "terrible beauty" is born. This oxymoron reflects the poet's ambivalent tone, as the speaker seems to fear and admire the transformation of "meaningless words" to action.  

The second stanza contributes even more to this ambivalence, as the speaker compares the rebels to stones that remain steadfast in their cause in the midst of a world that seems to be vacillating or changing "minute by minute." The speaker cannot resolve for us whether this steadfastness is good or bad, but it does set the uprising's leaders apart from the world in which they live.  

The last stanza refers to the sacrifices made by these rebels. Is it enough to "know they dreamed and are dead"? Were they excessive in their love for a cause? Were their deaths "needless"? The poet will not say, but he does go on to name each of these reactionaries—MacDonagh, MacBride, Connolly, and Pearse, acknowledging with respect that their "excess of love" for their country led to their deaths, but questioning whether their sacrifices were necessary. Did these revolutionaries inspire others with their deaths? Will more "needless deaths" occur as a result? The impact of the uprising and the resulting executions has yet to be determined. Perhaps all that can be claimed at this point is the ambivalent declaration that the Irish people ("whenever green is worn") have "changed, changed utterly." 

I included some links below that provide a deeper and more thorough understanding of the events that led to the emotions expressed in the poem.