I still stand by my original statement that it's immoral to cause some people to unjustly suffer to alleviate other people's suffering. If there is really no other option it is better to let them die.
Let's say that a portion of the population are at risk for a cancer that is not easily treatable and has a high fatality rate. This can be prevented by vaccines, however, it is only effective (e.g. herd immunity) if those who are not at risk of getting the cancer (it is easy to identify these individuals) also get the vaccine. Should the vaccine be mandated? This means that the choice are 1) Mandate that everyone gets the vaccine, with the worst side-effects being slight tenderness near the injection location or 2) No vaccines administered, but higher rates of cancer.
Given your stance, it would seem like you would consider the suffering to be an unfortunate natural side-effect and the means to prevent it would be objectionable since it poses the slightest amount of pain to individuals that don't benefit from it. I don't understand where this is coming from since, this is basically the equivalent of giving blood, which is typically considered a morally noteworthy action. In both cases, you get a pin prick, and then someone else gets a renewed life.
What about this situation: 2 people are eaten alive by lions and this gives much pleasure to 100,000 people watching. The net utility should be positive. Wouldn't Utilitarianism say that this is OK?
Remember, it is about the totality of experience. Killing someone is such a great detriment not because of the pain leading up to the death, but also because it eliminates all of the positive experiences that they would have experienced (for a young person this would be accumulated over decades). First, consider that the pleasure by the spectators would be measured by minutes. Then consider that the expected time that a 27 year old is conscious until death is approximately 300,000 hours. Even thought the crowd outnumbers the participants by 50,000 to one, it is easy to see how they don't overwhelm the utility calculation since it is actually the participants that dominate the calculation. However, you could overcome this by arbitrarily increasing the number of spectators to an unrealistic amount.
This is also assuming that nobody worries that it could be them in their and have the displeasure of fearing for their life on a regular basis. This is why a community of Samaritans is desirable; when someone finds themselves in need, they can expect others to help them, not cheer for their death. It is preferable to have this security, which is why such a society would not partake in gladiator events. Also to note, this also means that such individuals would not take pleasure in the death of another, they would feel sadness.
Also, this hypothetical stipulates a society that is full of individuals that get pleasure from the sight of others in pain. What is the consequence of this? Presumably, it would be rife with citizens committing acts of violence against others. If we were compare countries in which have such citizens to those that don't, which is preferable? It doesn't take much effort to determine based on those consequences that we would rather have a society in which people don't take pleasure in the misery of others and we work to shape those who do. Therefore, it would be obligated to implement this societal change.
Also, you need to look at the opportunity cost. Can they accomplish the same amount of pleasure by watching those same participants doing something that doesn't require their death? We have effectively replaced gladiator matches with boxing. Boxing results in higher utility than gladiator fights (same good effect without the bad), therefore utilitarianism would prefer boxing over gladiator events. This simple lesson shows us that if we can eliminate the element of death and retain the pleasure aspect, we should do so. If we have an option that produces more utility, then it becomes the morally obligatory choice, and the lesser one is ignored.
For the above reasons, I highly doubt that utilitarianism would advocate such actions and for some of the assumptions, utilitarianism advocates for the opposite which renders the hypothetical null. It is not a coincidence that moral progress is correlated to expanding the set of individuals of who we are empathic towards. This means that the main goal of a utilitarian would be to take actions in order to make sure these assumptions are not met, and if they are, to undo them.