What we saw in Ultimatum 5 was so utterly random and unmotivated that Doom's death just cracked me up. It was the very definition of hilariously bad writing. We don't even see the jet to let us know how Ben got there, or how Ben found Doom-- the scene comes off like Ben just walked across the damned Atlantic Ocean until he bumped into Doom and was all "Hey man, I'm tried of fighting you now *SQUISH*"