i considered Cap, but it was just a hand, no way to tell if it was a man or woman
i'd still go with Black Widow for now.... especially since it was Hawkeye that was hit.... you remember that they were introduced together in the office block with the aliens (at least i think they were intro'd together)
Traitor has to be Cap or Nick fury, note that he says 'soldier' in addressing Hawkeye. No one else on the Ultimates talks like that. BUT, in Ultimate war I think it was, Proffessor X was talking to Nick Fury and he addressed him as Mystique, if I remember correctly.
Well so far Ult. Reed was talking to another Reed for quite some time. The other Reed even said that your Avengers are called the Ultimates. But when Ult. Reed went to crossover to the apparent 616 universe there seemed to be a problem cause when he arrived there was nobody around. Then the comic ended.
Ultimate Secret only has two issues out so far.
Nobody knows yet.
"I am not afraid of the dark the dark's afraid of me"
Just thought I'd point out that Hawkeye's not dead. Remember who forgot their purse at Hawkeye's house; Wanda. Quicksilver's on his way back to pick it up. He'll be able to rescue Clint, and stop Cap (the assassin judging by the cover of the next issue). I'm willing to bet Cap's been through some type of brainwashing by the government when he got his powers. (You wouldn't want an independent thinking super soldier now would you?)
Cap is too obvious though. Look at the traitor's hand size when making coffee and talking to Hank. It's small and slender and a person the size of Cap (6'3", 280lbs) would definitely be bigger than that. Besides, Ultimate Cap might be a badass but he's still a boyscout at heart. I have a hard time believing that a guy who beat up Hank for hitting his wife is going to go and kill someone's wife and kids.
I'm confident the traitor is Iron Man. For the one, its unexpected. Two, the traitor is tech savvy. He leaked the Hulk's real name without leaving a trace. Cap can barely use a remote control. Three, one of the writers mentioned how Stark's line in this scan would play in a significant part in Ultimates 2.