I don't understand the apparent rush in this thread towards relatively high-tech devices requiring reliable 20th century triggers etc, or superior explosives.
So its the 12th century and some rebellious group begins to recruit fanatics to blow up the local market belonging to hated rulers. They take a wagon, put a barrel or two of gunpowder in it under some other goods, and have a fanatic drive it in. He gets there and sticks a torch (or whatever he might have, a lamp maybe) into the "package." Boom! History's first terrorist attack.
The point of these sorts of attacks, along with the terror, is that they can be delivered against powerful opponents with very little logistics or resources. There isn't even a need, in this off-the-cuff scenario, for much damage if people are afraid to go back to that market.
My point is that if you are looking for the earliest potential suicide bombers, it seems to me that you should be looking much further back.