From a legal perspective, I think the matter is unclear. There was a discussion whether to undertake certain criminal acts (assassinations), and there was apparently a vote not to. I'm not sure that adds up to a conspiracy. Kerry then, apparently , resigned. OK, even if there was grounds for him to believe that others might continue to conspire, he withdrew. The tricker question is whether he then had a legal duty to turn them in. I think the issue is sufficiently fact driven and hence murky that it would have been very difficult to secure a conviction here.
What it says about Kerry's judgement and responsibility, is different question.
I wonder what the names of the seven Senators were? And if they are alive what they think of Kerry now. Just a curiosity question. I think Kerry had a legal duty to turn them in since he used all of this to further his political career and all the time had higher aspirations for the presidency.