Netflix has put more seasons of Spooks on Instant Watch, much to my delight (though for some reason, Season Five isn't available, which is rather vexing). I still think one of the best summings-up of the show comes from the Twitter feed of drsamueljohnson, who declared it an "Altar on which a patriotick Mummer-Troupe does save Britain by frequent Sacrifice of its attractive Members." The frequent deaths of key team members on Spooks makes sense from both a plausibility and an artistic sense: the willingness to kill anyone who goes into the field illustrates the danger of the work involved, and it serves to dramatize the fact that the show's bureaucrats, like Harry and Ruth, are in fact the most important members of the operation. But I do think the show raises important questions about circumstances under which it's effective to kill characters.
I think George R.R. Martin and Joss Whedon are both instructive cases to consider. Both use character deaths to illustrate the consequences of the enterprises their plots are concerned with.
Whedon has a bad reputation for killing beloved characters, but he tends to use sacrifice of sweet souls, the closest things he has to innocents, to emphasize the cost of choices the people dearest to them make. Tara's death is a vehicle for Willow to go completely over the edge, but she's also killed because there are consequences to a world where magic and the laws of behavior are increasingly uncontrolled. Buffy could be just as dangerous as Warren if she wanted to accumulate power (Faith isn't that forward-thinking, I think), but having him accidentally cause his girlfriend's death isn't enough of an illustration of that. Having him commit murder when he's thwarted and angry is a more overt illustration of his unpredictability, anger, and power, and having him kill someone strategically marginal but emotionally central is a great illustration of his malignancy. Zoe and Wash want to have a normal marriage, but they've chosen a way of making a living that makes that essentially impossible. I agree that killing Wash the way Whedon did in Serenity was poorly planned, but I do think that ending their marriage was a useful dramatization of the pain that was an inevitable result of the gap between their hopes and their choices.
Martin, on the other hand, describes scenarios in which everyone in society, from kings to baker's boys, is in fairly serious danger of violent death. That makes the death rates of major characters in A Song of Ice and Fire actually rather reasonable, where I'd guess the death rates for characters in Spooks are actually somewhat high for covert operatives (though I'd be open to corrective if folks have better information). The problem is more that he's tended to undercut those deaths: characters we thought were dead turned out never to have died at all, or to have been brought back from the dead. At this point in the series, no death can be trusted to be final, or even to have occurred, so death has ceased to have the impact it had earlier in the novels (except in some rare cases like the Red Wedding).
In other words, fictional death is a lot like fictional sex, I think. Each death, in each circumstance, is different. There's nothing wrong with killing one character or many as the narrative demands. It's just a matter of getting all the impact of each death right. Taking a life may not be as consequential on the page or screen as it is in life, but if you're going to kill someone in fiction, you ought to do it with mindfulness and style.