In short, even mature and democratic societies don't stop people being greedy and thinking they deserve symbionts. Verad wouldn't be the only one willing to kill for the social and mental advantage of Joining.
Presumably, having a stolen symbiont would be neither socially nor legally acceptable. It would be a difficult thing to hide, especially if you wanted to enjoy most of the advantages of having the symbiont, and anybody who stole a symbiont and was caught would have the symbiont extracted, resulting in the death of the host who stole it. Unless youíre planning on going into hiding in the Gamma Quadrant, I donít see any point in stealing the slug.
On that subject, I kind of wonder what Verad Dax was planning on doing after leaving DS9. Going into hiding in the Gamma Quadrant might make sense for Verad, but itís horrible for Dax. If Verad dies in the Gamma Quadrant, Dax dies too, for lack of another available host. What happens when the interests of the host and the symbiont are so much at odds? How does the combined being make the decision?