Merits abuse is something that is not investigated very often, but even when it results in negative feedback, it does not mean that the account is automatically destroyed - it used to be like that, but today you have examples of accounts with negative tags from DT1 members participating in the sig campaign without any problems.
That's because the user has a long forum background and has a lot of positive feedback. Sometimes, users get negative feedback just because of disagreement, and we have seen that a couple of times. Sometimes, managers do not care about the negative feedback if the user is worthy enough to be accepted in a campaign.
There is actually a way to find out who gave you negative karma, and I thought it only required payment, but now I checked and I see that it also requires some luck. That option is called The Eye of Sauron - See who gave you karma
Yeah, I know that. But if I am not wrong, you cannot use The Eye of Sauron to check other members' karma stats. You can do it for your own account only. Also, as you mentioned, you need some luck as well.