It seems that you are asking about psychological manipulation. It is, indeed, a powerful tool but feats like you are describing are close to impossible:
able to manipulate people into committing suicide with just a conversation
This is only possible if the manipulator deals with an already suicidal person who just needs one final push. A mentally healthy person is very unlikely to commit suicide since survival is a very strong biological imperative.
or plunging a town into chaos by driving the townspeople into killing one another through paranoia
This is not entirely impossible, but it would require a town with at least two distinct cultural groups with a long history of mutual hatred and prolonged efforts to sabotage the existing 'peaceful' status quo and plant paranoid ideas into the heads of town inhabitants. It may take years to turn occasional brawls into full-scale warfare.
Psychological manipulation requires a very good understanding of other people, their emotions, their value systems, and their vulnerabilities. In addition, a person being manipulated should be somewhat lacking self-awareness and be in a vulnerable state. It is extremely hard to manipulate psychologically mature people who live fulfilling and stable lives.
The main limitations for psychological manipulation are:
- cultural and linguistic barriers
Culture and language influence our thinking, perception, emotions, and decision-making. It is harder to manipulate people who belong to a different culture and receive manipulative messages in a non-native language. Bilinguals also have higher resistance to external influences as opposed to monolinguals.
- personalities of people being manipulated
As mentioned before, psychologically mature people are hard to manipulate. Other factors that can increase resistance to manipulation are high emotional intelligence, satisfaction with one's life and personality, cold and/or detached attitudes, hard personalities, no previous psychological trauma, effective coping mechanisms.
A short conversation may be enough to goad someone into buying thing they do not need. However, even the most skilled manipulators will struggle to achieve something bigger in such a limited time. The bigger goals of the manipulation the more time is required. It will take months or even years of sustained effort to convince an entire town to do something they haven't planned already.
- resources that a manipulator has access to
It is impossible to manipulate big groups of people without access to significant resources. These resources may be human, financial, or social. You have to have a well-oiled propaganda machine to convince the entire nation to commit atrocities.
- social norms and institutions that resist manipulation
Manipulation on a grand scale is only possible when society fails to protect its members. Different societies implement different protective measures. Some examples of these measures include term limits for politicians, recommendations to seek multiple opinions, mandatory cooldown periods, limits on deposits, various protection and investigation agencies that deal with cults, abuse, etc., laws against hate speech, self-censorship by media, and alike.
Theoretically, an extremely skilled manipulator can manipulate the entire global population. It does not mean that every single person will be manipulated, some people may keep clear heads and see through the manipulation. However, the situation will be outside their control. Additionally, manipulation on this scale will require years of preparation, this cannot be done in a day or two. Also, there is no guarantee that it will be sustainable in the long run.
