i love the idea of a pure stealth game where killing is an option but, really, you are awarded for not because the moment the whole household wakes up in the morning everyone will know the best jewelry of the house is gone and so a manhunt/posse is arranged... and every low-life, hobo, etc who lives in the underground of the world you frequent will almost certainly guess the best thief in the area - ie, YOU - will have done it, thus, making your chances of doing another burglary highly unlikely or 1000 times more difficult.
however, some guard who was KO'd by sleeping draft, etc may be too embarassed to admit he nodded off for several hours after a half mug of ale so several days may pass before the lady of the house screams that her maid has stolen her priceless gold 'bridal belt' because she is the only one who knows of its whereabouts. perhaps by then she may realise that a lot more of her 'treasures' have gone missing, and in the intervening time you have had time to rob every other rich sod who's worth robbing from without them ever noticing.
if you murdered a guard/maid/etc during your burglary then it's likely you - being known to the underground of the area - will eventually be snitched upon and you become suspect number 1 who is facing not only aggravated burglary but murder with intent also, which would certainly add a most horrible death penalty to your sentence after sitting in a jail cell for 10 years, eating rats and drinking your own piss to pass the time.
killing people in games is just the lazy way out it seems to me. a REAL thief would've learnt the consequences of the 'Bill Sykes' type of thief [from dickens' 'Oliver Twist'], watched as his body swung in the cold morning breeze, constantly pecked at by crows, ravens & any other animal willing to risk the climb for a tasty titbit of human flesh, and thought, throughout his childhood as an urchin stealing apples and spuds, then on to more lucrative fare, every time he'd been caught he'd have been whacked or chased then forgotten.. but KILLING people?
of course, when there's absolutely no choice? then you'd have to. but you'd make sure to hide the body so it lies hidden for at least a day or two.
ultimately i spose it depends on who you're stealing from but if it's a sandbox game then 'knowing' who you're stealing from is a critically important factor. burglarising the major is risky but well worth it for loot, but kill his servant and, then, a day later, finding out his daughter has gone missing, then found dead.. [so she WASN'T a servant after all?] will certainly have a lot of 'fencers' [fencers are people who often put 'work your way', for those not in the know.. and they do it because they want the loot you're about to steal so you don't spend time popping around every pawn shop asking 'how much will you give me for this diamond encrusted platinum necklace?', looking like an old tramp who found it on the street, but the pawn store owner knows full well you didn't just find it] dubiously glaring at you, wondering if they can get away with selling you out to the law for a tasty reward whilst still only giving you a few coppers for the 'fake' shite you're 'trying to fob off on them'. ie, you have become too hot to handle. you're a dead man walking. and anyone with sense will avoid you like the plague in case they end up hanging from a meathook in the town square.
my final [and, as usual, long winded] point is that 'thieves' are NOT 'assassins'. killing is a fine art [i'm guessing] & tho the skills used for 'killing' & 'thieving' are very similar, it is very easy to forget that a thief has rarely done nothing more violent than clocked someone on the noggin. if he killed that person then he certainly didn't intend to and most likely didn't think anymore about the guy he just accidently killed by crushing his skull with a billiard ball in a sock because he rushed past and ducked out of sight never to be seen again..or something.
but an assassin? well... it's HIS JOB to kill. he probably even enjoys it. taking a human life - and in such an ignoble and often wicked way - is his skill. to leave evidence around to make it look like someone more likely than a hired killer did it is also his job.
when the thief kills, it's likely an accident [and he may just pay the consequences] but when an assassin does it, he has thought about it, planned it, so that nobody would ever think it was planned by an assassin but more likely the person related who's on his will or some such ploy.
more than anything, it'll stop the frigging lazy sods who just barge into quests slaughtering everyone thinking what fun it is and wouldn't it be great if they could walk into the office in the morning and do it to all the bastards on £200k a year on the floor upstairs!
i fank yew i fank yew