Assuming for the moment the magic of the death note prevents me researching and reverse engineering it in any way:

I'd research the people who's death is most likely to result in positive outcomes and kill them. Off the top of my head I'd go for current dictators and their immediate underlings. For example right now killing Robert Mugabe and the upper echelons of Zanu PF is probably the best thing that could happen to Zimbabwe (at time of writing he has just 'won' an election and the opposition are already mobilised, so a slight push is all that is really needed to collapse the regime).

Ideally, if I could ensure suitable anonymity protections I would publicly declare my intentions to have them killed in such a way that identifies me as the killer (e.g. send media outlets a statement with the exact time of targets death). Once my threats have be shown to be sufficiently reliable I will start making them conditional, giving myself the ultimate political blackmailing machine (e.g. if the international Red Cross does not have credible evidence within 30 days that all detention camps in North Korea have been closed and prisoners released, every member of the people's congress will die simultaneously). Assuming I can maintain my anonymity in the long run I would be able to do a significant amount of good.