Is the act of providing potentially harmful things to others unethical from the utilitarianism perspective? Would it be immoral to serve things that definitely will harm people (cigarettes, toxic food etc) to people who know of its effects yet still are willing to consume them?
We knowingly, routinely harm ourselves or increase the risk of harm to ourselves in a number of ways, such as (in no particular order):
We fail to exercise as much as we should,
We consume food and drink of type and quantity that is less than optimal,
We engage in a wide range of other risk-taking behaviours, such as sport, war, employment, driving, procreation, drug (including alcohol) use... Almost everything we do - including normal social interaction - entails physical and/or psychological risk.
With this in mind, to suggest that a company who provides a means of risk (such as alcohol or nicotine/tar, or perhaps gambling), is unethical merely by fact of offering such a product may be hypocritical.
This may be contrasted however with companies who, knowing they provide a risky product, do so in a manner which leverages the dangerous aspects of the product to entice more participants in the risk (without disclosing the risk, or via concealing/fraudulently diminishing the claimed risk). This might be deemed unethical where the former is not. As an example, consider the gambling/tobacco/alcohol company who manufactures products and environments which are designed to augment the addictive component of the experience (such as via adding nicotine or utilising addiction as a design element of the gaming experience). The addictive component generates profits by working to diminish the self-restraint that a person might ordinarily exercise in an attempt to minimise self-harm. This is done for the profit of shareholders at the expense of the customer. In utilitarian frameworks wherein utility is defined by wellbeing, in consequentialist frameworks, wherein the consequences are unnecessary harm, deontological (duty-oriented) frameworks, and virtue frameworks (where virtue is obtained via the minimisation of unnecessary harm) such behaviour might reasonably be deemed unethical. Under an extremely libertarian ethic however, in which the autonomy of the individual is highly valued - including the freedom to become addicted or to persist in addiction - knowingly providing addictive products might be not merely moral, but almost obligatory, in that they are providing for an extant desire.
This is where it gets tricky. If there is no heroin dealer and no means by which to make heroin as an individual, then providing heroin, an extremely addictive substance, would be introducing a desire and a risk that did not previously exist. However, to join a pre-existing market of heroin dealers might possibly be deemed as ethical if the product is of higher (healthier) quality and derived from ethical (eg: non-violent/exploitative) sources.