If the British killed all the rebels in the shelling they might be seen as having brought it upon themselves. The 1916 rebels were not initially at all popular in Dublin. It was the brutal behaviour of Captain Bowen-Colthurst (including the shooting of Francis Sheehy Skeffington, a well known figure who although a conscientious objector was attempting to restore order and prevent further looting - the first example of an innocent victim caught in the crossfire in the Troubles in Ireland) and the subsequent execution/martyrdom of the rebel leaders -and McBride who wasn't actually a leader but the British were convinced was because of his actions during the Boer War, that outraged public opinion -even Southern Unionists didn't like it. If the rebels had all perished without subsequent executions or with all survivors found guilty having sentences commuted to twenty years hard labour, the Easter Rising could have been a footnote in Ireland's history.
Realistically though, the British largely applied the letter of the law even though it would have been politically expedient not to do so. Probably the worst thing they did was to try and hush up Bowen-Colthurst's activities. Actively committing atrocities would have needed a very different commander to Sir John Maxwell. With all their faults, the British Army of 1916 wasn't Santa Ana's Mexicans (the funniest bit of the whole rising was a local cease fire to allow a park keeper to go and feed the ducks!) If Bowen-Colthurst hadn't been about and wiser political counsels prevailed re the executions, the whole thing might have died down, especially if Conscription had been ruled out.