Of we're talking points, then you're rolling the dice. If they've already murdered, you're not preventing those, so you've done no good. If they're going to murder at least two more people, you should net out positive, by preventing those murders. But you can't know they'll murder more people; maybe their murderin' days are over, and they've given it up; maybe they'll get hit by a bus before they can kill anyone else; maybe they'll get caught and imprisoned before they can kill again. If you murder them, but they'd never have killed again anyway, you're pretty well net negative.
Very mild spoilers if you haven't seen Season 1 of The Good Place
Although, The Good Place is ambiguous about how intention impacts points. Take Tahani: she's there because, despite all the good she did, she did it all for the wrong reasons. OTOH, take Doug, from S03E08. He did everything he did because he had an epiphany that told him exactly how the system worked, so everything he did was to maximize his points. By the Tahani rule - and by the plot device of several other episodes - having that knowledge taints your actions and prevents you from gaining points from good deeds. Yet Michael pretty clearly believes Doug is the template for how to get to the Good Place - a direct contradiction of - if not Tahani - than other episodes where the characters are doomed because of their knowledge of the system.
I've only watched through season 3, so if there are any other spoilers below, they're purely accidental.
So: while The Good Place is somewhat ambiguous about the question of Doing the Wrong Thing for the Right Reason, I think in balance it'd weigh against you. You should have tried other things first - like tipping off the police. If all you're trying to do is get into the Good Place, your best bet is to try and reform thre person. Even if they killed you - maybe especially if they killed you - self-sacrifice in a good cause is clearly a lot of points.