But I kind of suspect that most of ISIS' victims are heavily drugged before they are executed.

I believe that because

a) These guys seem to put a lot of effort in their video productions and a flailing/fighting victim means they risk something going wrong at their expense, making them look incompetent for example.

b) I wouldn't put it past them to believe that by drugging these people, the victims are guaranteed not to get to heaven?

I mean they are psycopaths so anything is possible.