Yes the attendants did kill a holy being, but the result was a peerless dharma protector has come to tame the world, how wonderful is that! So comparing the negative karma they have created is small compared to to the MERIT they have created, this might in the long ru have quickened their path to liberation.
I recall Shariputra was asking why did Manjushri taught emptiness to 500 monks who were not ready for it hence they developed wrong views and many went to hell, but the skilful means of Manjushri to quicken their path out of samsara. I felt as before the attendants were already heading to hell, nechung just manifest the conditions for the attendants to act it out, but it coud be any other condition later that triggers these attendants heading to hell.