Kadri and Havelocke, kindly stay out of this thread. Or I'll kill you. Again.
I'm going to give the background of my question first, those of you not interested in the specifics of my campaign can skip to my question at the end.
I've been running Dark Heresy for about a year, though my sessions have been pretty infrequent with school/work/one player living on the opposite coast of the country. Until recently, I haven't killed a single pc, I hadn't even managed to cause more than a point or two of critical until the session before last, nor was a single point of fate burned. The only fatality we had was the resident evil-insane psyker going Daemonhost, which I had hoped would be a crazy hard boss fight, but he was dead within two rounds of his arrival. The cell consists of an untouchable metallican gunslinger, a forge-world assassin, an intellectual, combat-ineffective tech priest, a feral world guardsman/mutant, and a schola progenia arbitrator, all around 4,000 xp. Most of the gunfights I sent against the pcs were quick, bloody affairs, with the Metallican gunslinger and sniper-assassin blowing away all opposition in under half a minute.
That all changed recently. After a fight with a Goleph in a sewer complex, the gunslinger was at something like 7 critical. They killed the Goleph, most definitely, but had to abandon their larger investigation to help the gunslinger. Then, last session I had 3 fate point burnings in rapid succession.
First, the pcs were breaking into the private shuttle of an Interrogator aligned with another Inquisitor. I had two bolter-armed Oathsworn body guards stationed there, one of whom managed to nastily frag the feral-world guardsman before buying it. The Feral worlder had to burn a fate point to survive, though since he's also a necrophage he was back on his feet fairly quickly. Then, the group wound up in a fight with 5 Oathsworn, plus a sollex-energy blade wielding noble. They managed to injure or incapacitate most of the Oathsworn fairly quickly, but then three pcs decided to gang up on the noble. Once they had slaughtered him, one of the Oathsworn fired a burst at them... killing all but one. Everyone burned fate points to avoid actual death, and some npc deus-ex-machina later all of the remaining baddies are dead.
While the forgeworld assassin has been cyber-resurrected, the untouchable gunslinger is basically out of it. His body has been torn apart twice in a row now, and I've officially had his barely living carcass turned over to the Assassinorum, to see if they can salvage him/turn him into a culexus assassin. While the player who was running the gunslinger is happy to switch to a pc who can actually talk to people, he has expressed the desire to get his old character back at some point in the future, which will be kind of impossible if he does become a full-blown culexus assassin. I've got a few outs (his body being stolen by the heretek they've been trying to track down, wackiness involving clones and mind-cleansing, all sorts of other stuff), but currently I'm a little worried by how often I've been slaughtering pcs.
Basically, this is a very long winded post asking for advice on how to kill characters properly. When is it inappropriate to kill a pc? When is it inappropriate to let them live? Should pc deaths always be something worked out before hand? What's your take?

This is not good.