I have struggled with Motive, Opportunity and Means for some time now.
I don't think people always need a motive. I think all they need is the Opportunity.
How many people would keep a bag of money* or shoot an ex* if they thought they could completely get away with it? Most of us wouldn't, since we follow moral codes, but there are those of us that would do so in a heartbeat if there was no fear of being caught.
*=examples only! I don't suggest anyone actually do these!