User blog comment:B214/Volume 22 spoilers/@comment-29778526-20170126060633/@comment-9595671-20170126071344

Well they're not trying to kill it, they're sealed with it because this seal seems to have Trihexa still aware and mobile, without them inside... it might break out sooner than they'd like.

For why they're alive? I think Issei's flames may have already done some damage so it's kinda exhausted and it's core shot out, and I think it's kinda implied without it's main body, the core's not at max power.

I may need to re-read it cause the way it was described was kinda confusing to me.