Come now, killing humans isn't that bad, you just press the button then they squirm a little, make more or less noise and stop being a problem. Nothing to get upset about really ...
... why yes my characer in our "1930" campaign was a hitman, how did you know ? ; )
His survival was due to a few things
1) He took upon him to selflessly stand guard for the group, which meant he would shoot things from afar rather than going up close with every horror we encountered. The best defense is to stay out of range from the claws/fangs/tentacles/etc.
2) He preferred to let others get a good look at the cursed tapestry, the maddening tome or the unholy scrolls. In case they got possessed, driven murderously mad or anything, it combined well with point 1).
3) While caution won't save you from an awakened Great Old One, it does help against the more "mundane" abominations. In general it is a lot easier to rationalize things and keep calm when you see them from 200 meters away rather than while being held by their slimy appendages, feeling their reeking breath on your face and being seconds away from being eviscerated by their bilious talons.
For example :
Our group of wannabe cultists find themselves in south east asia travelling north toward an important objective when in a village they learn of the existence an ancient crypt which is rumored to be cursed... with the undead !
Of course, the mad doctors, archeologists and other supposed intellectuals of the group are very excited and vote that we look into it, despite it having nothing to do with our main mission. After a few hours trek in the jungle they find the crypt and a staircase that descends into its depth. Futhermore, examination of the glyphs reveal that the entire graveyard might be under the effect of a necromantic spell.
Donald Jacobson, the hitman, announces : "Well ... I see a ridge overthere which would be a perfect position to shoot from in case ... you know ... an army of skeleton rise up from the surrounding graves, lusting for the blood of those who dare defile their final resting place for example ... Surely you see the wisdom in having someone making sure you have a clear escape route from the cursed crypt ?"
He even convinces the others that Elizabeth, the cute red head ex-barmaid he had a secret crush on, should also stay out to provide fire support. They take position while the other five enter boldly into the crypt ...
After a narrow staircase they follow a long corridor leading them into the darkness. All the way they encounter crude sarcophagi of rotten wood lining the walls. They hesitate ... Should they open them ? Destroy them ? Would that... awaken the dead ? They decide to let them be, surely exploration is the best thing to do. Looting and destruction can wait...
Finally they reach a large round room and torchlight reveals, in the middle, a tall throne and its occupant. The skeleton wears the worn remnant of a rich garnment, a large onyx ring is wrapped around his index and his hands rest on the dusty pages of an opened tome in his lap...
Henry, the doctor, smiles and he leaves boldly the tight group to approach and take the tome from the skeleton... who, despite having no sinew or muscles, doesn't let go of it. He doesn't have much time to ponder that strange fact before the owner of the book seizes his throat, bony talons piercing his flesh and streams of eldritch imprecations flowing from his gaping maw.
Around them and in the graveyard above, the dead rise at their king's command. Horrified, the group tries to back up into the corridor while Donald and Elizabeth, from their vantage point outside the necromantic perimeter, open fire to prevent skeletons in the graveyard from entering the crypt.
Igor, the big asylum warden and trained brawler tries to take the lead, using his talents to dismantle the "lesser" skeletons clumsily trying to grab him. Unfortunately there are too many of them for him to make real progress. Behind them, Dr. Henry falls on the ground, his severed throat having remained in the King's hand.
Terrified, George, the archeologist pulls out a stick of dynamite from his duffel bag and lits it on his torch. Did someone bump into him during the chaos or was his hand shaking too much ? The lit dynamite slipped from his grasp, fell at his feet, rolled outside his frantic reach and blew up, killing all but one in the blast or under the falling rubbles of the collapsing corridor.
All but one, indeed poor Jeremy, the engineer, survived George's initiative and he was left wounded, alone, in the dark, walled in by tons of rubble and in the company of the irate undead King...
Thus only Elizabeth and David made it back to London, their mission aborted but giving their chance to a new group of cultists pulled from the ranks to fill the slots and prove their worth !
Ok so it wasn't a total party kill nor a campaign end but 5 / 7 killed is not too bad, right ?
PS: On a side note, I must learn to summarize my posts