Hmm, as I recall Six clearly got the drop on Shockeye, how else would he have got close enough to stuff a poison soaked rag in his gob? Taking that fact into account there were a whole heap of other options open to the Doctor, from whacking him on the head with something (which ok might have killed him) to using his own physicality (it isn't like we haven't seen the Doctor handle himself in a fight before now, what would Three have done I wonder) he could even have carefully dosed the cloth to render Shockeye merely unconcious. But no, he clasped the cloth to his mouth and held it there until he was dead.
Now don't get me wrong, Shockeye was a terrible character (in a terrible story) but there was always something rather creepy about that, and whether it was self defence or not (the nasty man cut my leg!) the Doctor still killed him (and by the way I think you'll find self defence won't always excuse you of murder).
This doesn't mean I think what 11 did was right, but this isn't the first time the Doctor has acted in this way and likely won't be the last, and I get the impression that what he did to solomon won't just be forgotten either...
