Something happened that wasn’t supposed to. Stevens was not supposed to be killed.
There are two main possibilities.
First, Stevens was supposed to be kidnapped and swapped for the blind Sheikh. The “attack” on the residence was a sham, until the Seals showed up and turned it into a firefight. The house caught fire, Stevens was trapped and died of smoke inhalation.
This scenario, though, would not explain what happened later that night.
Second (what I think happened) was that the attack was always directed at the “annex”, in order to seize whatever was in the “warehouse”, and the attack earlier on Steven’s residence was a feint, to draw security away from the “annex”. Things got out of hand at Steven’s residence because of unexpected resistance, and he died accidentally.
The “attack†on the residence was a sham, until the Seals showed up and turned it into a firefight. The house caught fire, Stevens was trapped and died of smoke inhalation.
It seems there were enemies running through the compound and boom they started dousing everything with the Americans’ own drums of gasoline. The operators were not there yet. The first thing they did was to loot and burn the Ambassador’s building.