More can of course be said, especially by someone who has read up on the Boxers in more detail more recently and can thus remember the details better, but as I note in passing in this past answer, the siege of the Legations was primarily an effort of the Qing regular army overseen by Ronglu, who was an anti-Boxer with reformist sentiments and seems to have been reluctant to prosecute the siege to the fullest extent.