Sounds to me more like he's producing embarrassments, not a degradation of national security.

Most of the info appears to be not confidential or at a low level. There's no top secret stuff among it.

Besides; would you want spies to be shot when caught? That would certainly kill scores of Americans world-wide.

Finally, Assange is not in the U.S.. An assassination in a foreign country could easily earn the CIA a status as a criminal organization in that country and possibly in others, too. The backlash could be enormous and very well exceed the benefits.
Oh, I forgot. There would likely be no benefits other than revenge. The Manning leak may prove to be a one-time event for Wikileaks, and they already uploaded all that they have (encrypted) months ago.