Based on my current understanding, it appears he attempted to fire a tear gas cartridge at Dorgan at close range. He was also carrying two knives. This leads to a number of conclusions, none making complete sense when viewed against alternatives. Most likely off the top of my head seem to be:
1) An extreme protest against Dorgan. Think Bush shoe incident times 10.
More likely, but also somewhat odd:
2) Tear gas seriously injures Dorgan and disorients the crowd. Stab to death in some dramatic fashion. What I don't get here is why not just have a pistol in the first place.
Another alternative is the would-be assassin was set-up, so that even if he got his cartridge off he would not kill Dorgan. This also makes less sense when you think anyone who would be willing to injure Dorgan would probably also kill him.
In regards to the assassin's complacency when subdued, yes he could have put up a better fight. But he may have been willing to simply accept his act, and also he very quickly starts getting many blows to the head. It's hard to say when surrounded by an angry violent mob how anyone would react, it seemed like he was resolved to curl up and protect himself.
link
http://www.guardian.co.uk/world/2013/jan/19/bulgaria-gas-pistol-politican