He worked very hard, devoted his whole life to qualify for being joined legitimately. Then, as he said, how whole life was reduced to one word: UNSUITABLE.
That didn't justify his actions, but they were understandable. Though one wonders why he didn't just find a joined Trill who had re-associated, and was sentenced to die with its host. The host dies, Verad gets their slug, no murder necessary.
I didn't think there was an execution carried out for re-association, just the Trill was exiled from the Trill homeworld and so when the Trill died of old age or whatever cause, the symbiont wouldn't have another unjoined Trill available for transplating in time. So Verad would have to find a elderly or terminal joined Trill and hang around with them and a doctor capable of doing the transplant possibly for years waiting for the Trill to die. Lot of waiting, and Verad didn't seem to be very good at waiting.
Besides, re-associated Trills seemed to be extremely rare because the sanctions were so strong. And not crazy about advertising themselves, either. It seems to me it would be like advertising oneself as having an incestuous relationship.
Lots of people have childhood dreams that don't come true. Kids want to be a doctor when they grow up, works hard in high school and undergrad, but still don't quite make the cut for medical school. Girl wants to be a ballerina and practices for 4 hours everyday from age 5 to 17, but then grows up to be six feet tall. End of dream. Most people don't become murders because their dream has to change.