Crimes of Opportunity are random events that spawn during regular gameplay. They appear on your minimap with a moneybag icon (like the trophy image) and task you with killing a few criminals and stealing their score from them. It's recommended to do these whenever you come across them as they can be a little annoying to make show up on command - you should find enough when exploring the city for other trophies but if you're not doing them you may need to spend extra time driving around aimlessly to find them. They do have the same spawn locations however, so if you need more after completing the other trophies you can inconsistently farm them by fast travelling away and driving back and they should respawn on occasion.

A few people have stated that destroying the armoured trucks that also spawn randomly on the minimap also counts towards this trophy, though I'm sure I would have earned this trophy a lot earlier than I did if they did count. I'm unsure if this is something that was patched out or you have to do something specific for them to count towards this trophy - either way it's worth destroying them anyway in case they do count (plus for a little boost in cash in the early game).