I was in the neighboring state's convention when this happened, and all conventions on Sunday and the next week in South India have been canceled.
Going by what my friends say, the bomber used to study with a JW and was a nationalist. He had created a lot of problems for the local congregations in the past.
He came to the convention dressed as a convention attendee, planted the bombs in tiffin boxes while associating with people there, and left as soon as the opening prayer began. Three bombs exploded within 5 seconds of each, and the resulting smoke made evacuation difficult. The body of the sister who died was charred beyond recognition.
In his video, the bomber said he was concerned about children and what JW's teach them, however, he chose to bomb a place where multiple children were present. Sadly, a 12-year old girl has suffered 95% burns and is on ventilator support, and 3 other children (ages 5-10) suffered 60% burns and are in critical care units.