It wasn’t about ‘letting’ them be murdered. Obama couldn’t send in an armed rescue team without alerting too many people to keep the mission quiet.
Everything was under control and could be explained until someone leaked that the CIA guys were denied help. That got too many questions asked to keep it quiet.
In other words, they let them be killed in order to keep the questionable arms transactions secret. “Everything was under control,” and Stevens and his defenders were dead, “until someone leaked” that they let them be killed in order to keep the questionable arms transactions secret.
I’m sorry, but you have just reinforced my conjecture that something else is involved here. Obama is an idiot, but not when it comes to politics. He must have weighed the possible fall-out, and decided that the arms transaction + X was too important to be revealed, and that it was better politically to take the hit for Stevens than to reveal the X factor. The arms transaction alone does not suffice.