Hey, I'll tell you the same thing I told my bosses at the Casino where I worked (and this has nothing to do with why I didn't work there).
I am not an implicitly honest person, in truth my integrity has never been tested because I haven't been in a situation where I would both profit enough to make the risks worthwhile, and felt I could get away with it. In such a situation I'd probably give in to temptation, but I've never been there, and probably never will be.
For me, my standard is simply that if I'm going to do something majorly illegal, the payoff is going to have to be enough to guarantee I can retire in relative comfort and never have to work again. The apparent risk isn't the only factor, any gamble that can put me away for a long time, or even lead to my death (one way or another) is going to have to be worth it.
I know a lot of stuff, and might be able to get away with killing someone, especially if there is no direct connection to link me to the victim (ie we don't know each other). I do not work as an assasin so it's not likely anyone is going to suspect that I am one. I have some forensics training so I'd imagine I can cover my tracks well enough since I'm totally unrelated to the victim (and contrary to TV, detectives don't work magic, a lot of the stuff that happens tends to be very hokey even with CSI units).
But see, even if it seemed "easy" the risks for something like that are substantial. There is no way I am even going to roll the dice for a mere hundred thousands dollars (or pounds if you'd rather). Oh sure, that might be nice for a couple of years, but the potential of a couple of years is not worth death row or life in prison if I'm caught. Not to mention the risks inherant in dealing with someone who would pay me to do that to begin with.
Truthfully if the "perfect" situation arose where I felt I could actually steal like 10 million dollars and get away with it, or kill someone for money... well I don't know if I'd give in to the temptation. Probably. Nobody can say what they would do in a situation like that until it happens, and frankly most people will never know.
Speaking soley for myself, I feel anyone who claims to be implicitly honest is full of it. As sad as it is I think when you get down to it everyone has a price for just about everything. The most anyone can claim is that they have never found out what theirs is.