Suder's an interesting character. Really though... on what level did Tuvok think melding with a man who admittedly has uncontrollable violent impulses... was a good idea? It's not like he even learned anything useful. Tuvok just reports to Janeway that... he's uncontrollably violent. What a revelation!
Also... gotta rant on this. Janeway's decision to lock Suder in his quarters? Seriously... Suder just randomly killed another crew member and Janeway tells him to go to his room? It's not like the alternatives are just morally repugnant to Janeway either. In future episodes she demonstrates she has no problem killing members of the crew when she thinks the situation warrants it -or- locking members of the crew into the brig for long periods of time.
There are procedures for trial cases without court personnel as demonstrated in Measure of a Man. It would naturally fall on Janeway to judge him... which being judgmental is no problem to her... but even if she doesn't want to do that... fine... put him in one of those nice stasis tubes the Doctor has until you get home then and a real judge can have a field day with him. Only down side to that one is Suder's gonna argue sleeping for x years as time served.
I think the real reason is Janeway's just too sinister to waste a valuable resource as a real live serial killer, so it may be handy to have him around in the future.
Also, the Paris arc... well there's a method to it all... him slowly turning into a screw up is kind of a subplot for the next few episodes.