I don't think that Nikola is only taking his medication because Helen doesn't want him to kill people.

Agreed. I've kind of changed my mind about that since I wrote this. :)

I do think he took it originally out of desperation, and then kept taking it, but every time he thought "Why shouldn't I be myself?" he thought of Helen, and kept taking it.

Of course, after he "dies", all bets are off for a while.