Aha, so now the question changes from:

how do you know that the killer believed that he would have access to 72 virgins?

to

did you mention the 72 virgins with regard to the Westminster killer?

Yes, I obviously did mention that as you have so keenly observed but the only one's who have mentioned anything about the jihadist believing that he would be rewarded with 72 virgins in paradise seem to be you and the Gnome.

Funny that.