While maxim 49 states that every target is one bribe away from becoming a client, a mercenary needs to remember about top priorities:
1. Get paid.
2. Live long enough to spend your money.
And it is not just about being a target for revenge - that's the standard risk in this line of work. It's more that the person who bribed you knows you have no loyalty other than money and he could be outbidded at any moment. Typically a solution for that is simple: off the risky asset once the job is done. And even if you live through that, who would hire you afterwards?
Anyone doing mercenary work longterm needs to be loyal enough that when contract is agreed upon, it is to be fulfilled as long as the employer is holding his end of the deal. Those who violate this kind of common sense I guess would not work or live very long.
Obviously, a throughout background check should always be made on all prospective contractors.