I'm wondering what happens to a lead score if they are already at 0 and they perform a behavior that decreases the score. Would the overall score go negative or remain at 0?
As Kenny said, it will. But it's not a good idea to let it go that way. If you add regular inactivity campaigns to you lead, it will end up with a high negative score. Then if suddenly, this lead becomes active again, it will take a lot of action on his part to just come back to 0. And when he eventually reaches the thresholds for MQL or equivalent, it might be too late.
At the end of the day, you risk to miss hot and good leads because of these negative scores.
I recommend that you set a trigger or batch campaign that detects negative scores and set them back to 0.
Agreed in less it's someone you don't want to score at all. ie. a competitor or employee - you should never score them so they don't QL.
Well in fact, you should have a "Keep Score to 0" smart list that includes employees, competitors, ... and that you would use as an exclusion filter on all your scoring smart campaigns.
I have a similar smart list that excludes employees/competitors from running through behavior scoring flows. It's solved a lot of issues for us.
I also use Score Token segments to decrease score for inactivity. For example, if their behavior score is less than 50 it will decrease by 5. Additionally, there's a filter that the behavior score must be greater than 0 to run through the flow.
Expanding on Greg's helpful tip:
Set competitors or applicants to -1000. That'll stop them from qualifying
Every so often, set the scores from -500 to -1 back to 0.
This way you'll give leads the chance to qualify, even if their score has deteriorated and the biggest negative scores will be for competitors and applicants.
The other thing to note is that if you use Sales Insight your stars and flames and best bets can be thrown off by negative scores. I've had cases with customers who had a lead with a negative score in their best bets and didn't know why. This is because according to the leads activity in relation to itself and other leads, that negative score was actually a good score, but it didn't look that way.