Locarno came up with the idea to do the illegal maneuver, and pushed the others into it. he then covered it up, and turned on wesley when he confronted Locarno over it, even going so far as to tell Wesley to resign from starfleet so that Locarno and the rest can get off. Locarno showed no remorse at all over the death, pushing the blame for it onto the others in the team rather than admitting he screwed up. it is heavily implied that he has been telling the other members of the squadron similar things.
in the episode we get a scene skip after wesley confesses.. it seems likely that the other members of the group, other than locarno, chose to back wesley up. and notice that in the end, picard does say that they all nearly were expelled.. and that Locarno argued in their defense. so Locarno, once he couldn't cover it up, fell on his own sword to protect the rest. but his argument was still very self centered, basically taking full credit for the whole affair, with no indication that he felt guilty over it.
while we don't know as many details for Tom Paris, we know that he had already graduated, and it was a bit of pilot error that led to the deaths of three other people aboard. no indication he was doing anything prohibited or illegal, just that he did something incorrectly and an accident happened. and his 'cover up' was a case of him refusing to admit that it was his error that caused the accident and a falsified report.. and then he came clean and owned up to it on his own, out of a guilty conscious.
this is why paris could be redeemed and Locarno not.
Locarno pushed others into doing something unsafe, then covered it up and pushed others into becoming scapegoats to save his own skin when the coverup started falling apart, all because of his own ego.
Paris made a mistake, but decided in the end to be a better person and own up to that mistake, even knowing it would mean being kicked out of starfleet.
