If I were you, I would stop worrying about the possible side effects as every drug has a long list of possible side effects, often grievous ones, but they happen rarely and often to the people who have some serious health problems.
Diabetes isn't really caused by drugs. It's a civilization illness, caused by obesity, unhealthy food, chemicals in our food and so on. If it worries you, you can regularly control your blood sugar.
Additionally, medication should be combined with psychotherapy in order to be effective.
Everyone should ask their doctor about side effects and risks, as they don't necessarily happen "rarely" - some of them happen as much as 10% of the time, which is quite often. Decisions about meds should be made by the patient, after obtaining information from their doctor sufficient to enable informed consent. Doctors have a duty to provide relevant information about side effects and risks. The patient's health is not necessarily a factor in whether and what kind of side effects they get. Side effects can happen to a person in near perfect health.
There are several types of diabetes, including type 1 and type 2, and they are very different disorders. Type 1 has many possible causes, and is not at all related to lifestyle factors or obesity. Type 2 is often related to obesity, but not always. Not sure what "chemicals in our food" has to do with it. Several classes of drugs, including some anti-psychotics, can induce obesity and cause type 2 diabetes. Previously, in developed countries, psychiatrists didn't pay much attention to this risk, but they have had to in recent years, because so many patients were dying from drug side effects such as heart disease and diabetes related complications. Psychiatrists in less developed countries may be a bit behind the times in this regard.
There's usually no need for a person to be on an anti-psychotic that carries an unacceptably high diabetes risk, as there are about 15 or more different drugs to choose from, as well as drugs in other classes. Not all anti-psychotics have this risk, and those that do, have it to varying degrees. Doctors can monitor the patient's weight and blood sugar level, and if either are increasing, they can review the drug choice. Type 2 diabetes is not just a simple case of controlling blood sugar - it is a complex disease with effects across a number of body systems. Best avoided, if at all possible.
In unipolar depression, psychotherapy can be effective alone or along with medication. In other disorders, such as bipolar and schizophrenia, it is less clear. In these disorders, medication is usually essential for managing the disorder, and psychotherapy can assist the patient with living with their disorder. It does not replace medication, in the way that it can sometimes do with mild to moderate depression.