Ultimately, if having to choose between feeding my family or feeding yours, the choose is obvious even to nice guys.
Especially when you consider that sometimes being nice to one person is being cruel to another.
The key is to protect yourself...when dealing with bad people you know where you stand, so you have strong motivation to demand contracts, etc...
When dealing with genuinely nice people, whom you would never dream could possibly screw you, thats were you are most vulnerable, because you are less likely to demand the contract and protections you need, and when it comes to them vs. you, they will often choose themselves...
Semi Related - I read a quote from Warren Buffet that really resonated with me, "Honesty is an expensive gift. Don't expect it from cheap people."