What level were these guards? (In-game, were they low on the totem pole as far as guards go? Were they appear difficult to fight?) If they seriously threatened the life of the ranger (i.e. NO other choice) attempting escape then he was, sorry to say, somehwhat justified to slay them in self defense (even if they were just doing their job).
However, in the strictest sense, if the ranger did not excercise every precaution and attempt to spare their lives (and yes even at greater risk, though not mortal risk, if neccessary). Then his alignment might be shifting a bit towards neutral now.
Like as not all the PCs disobeyed the law (whether they were afraid of corruption or not) and are subject to the consequences accordign to Starmantle. In fact it appears that the PCs walked right into their nemsis trap and did exactly what the bad guys wanted! Heck now they LOOK guilty! Now the corrupt evil folk can righteously track down and slay the adventurers now considered outlaws.
The only recourse I can see for them is redemption by completely unmasking the evil folk and their corrupt ways, but now it will be that more difficult to do so.
Bad ranger bad, if the rest of the PCs roleplay it they should be upset (perhaps sympathetic too if the ranger is remorseful) at his lack of self control. Either way not an enviable position to be in. Especially for the so-called ranger.

Hope these PCs are close (otherwise ranger seriously in trouble).
Interestingly enough let us know how it goes, sounds pretty exciting!