He originally defected because he considered the Romulans to have clear moral values and an absolute certainty about the purpose of life, something he called a "strict moral compass". After living on Romulus for twenty years he became disillusioned with the Romulan system. Their desire for conquest and the deception used for this purpose did not sit well with him.
Following his arrest, DeSeve insisted on speaking to Captain Jean-Luc Picard on an urgent matter. Picard agreed to meet with him, and relayed the message from Spock, explaining that a Corvallen freighter would arrive shortly in the Kaleb sector, carrying an important cargo that was to be transported back into Federation space. Picard considered DeSeve's request, after making a request of his own, to uncover DeSeve's motives for defecting back to the Federation. After due consideration, Picard ultimately agreed to DeSeve's request and set a course for the Kaleb sector.
Unknown at the time, however, the Corvallen freighter, which was transporting the cargo, was destroyed during its rendezvous with the Romulan warbird IRW Khazara. When the freighter did not arrive at its designated coordinates, Picard became suspicious of DeSeve once again, who himself did not understand why the freighter had not arrived.
Picard pried DeSeve for more information, only to learn that DeSeve had neglected to mention the type of ship to expect, what the cargo was, nor that he had not in fact received his information directly from Spock as he had earlier alluded to. DeSeve explained that, at the time, the information "didn't seem necessary," and that on "Romulus you learn not to volunteer information," describing it as "a hard habit to break."
After efforts to locate the freighter resulted in the discovery of its debris, and an anomalous polarized magnetic distortion. DeSeve speculated that the distortion may very well be a cloaked Warbird, and once the anomaly was discovered to be fact, revealed what he knew of Romulan tactics in these types of situations.
When the Warbird finally revealed itself to the Enterprise-D, and transported three Romulans to the Enterprise-D's bridge, via a disruptor beam, DeSeve was able to quickly identify the "cargo" as Romulan Vice-proconsul M'ret and his aides. (TNG: "Face of the Enemy")
Background information Edit
DeSeve was played by Barry Lynch, and was described in the script as a man "in his late forties, but his face is lined and worn beyond his years. He wears the unadorned uniform of an enlisted Romulan soldier."
Several of DeSeve's scenes and lines were cut from the final episode exploring his feelings of being "disoriented and alone" after returning to the Federation after twenty years among the Romulans.
Several other script notes described DeSeve's intimidation by authoritative figures, an attitude that was also reflected in several plot points during Troi's experience among the Romulans. This was best explained when Picard entered DeSeve's quarters and "DeSeve automatically snaps to attention out of reflex out the sight of a superior."
Noting the difference between Romulan and Starfleet rank structure, in another instance of his disorientation, DeSeve greeted Picard as "Commander" rather than Captain. Despite having previously referred to Picard as "Captain", when he made his request with Riker speak with Picard, DeSeve's later faux pas was explained by a cut line, where he had excused his error by stating, "Captains, not Commanders... it's been so long since I've... I'll remember."