I wouldn't say that those devising Utilitarianism are rulers as Jeremy Bentham and John Stuart Mill were the ones to do so and they weren't particularly powerful men

Alright, but they were probably acting on behalf of some rulers. But feel free to ignore that claim for now.

Utilitarianism is the principle that the only meaningful end in life (meaningful as it pertains to humans) is human wellbeing (happiness, utility, pleasure, etc.) As such, maximizing wellbeing is its central principle.

I've heard another definition of Utilitarianism where the idea is that the morality of an action is determined by its consequences. I guess those two are relatively close, because obviously for the action to be moral, the consequence would need to be perceived as moral too, and "well-being" would certainly fit that mold.

A central problem with Utilitarianism is that people act based on their perceptions, but the perceptions themselves are based on any individual's sense data, thought-patterns, pre-conceived notions and so on, and thus, they are subjective.

So if people are just going around pursuing "well-being", there's no telling what they might decide to do. Besides, what the idea of the morality of an action being determined by its effect on some sort of perceived well-being really boils down to, is the idea that the end justifies the means.

For example, I might decide that some exercise would be good for you, and chase you around with a baseball bat to enhance your physical well-being. I'd think that my end would justify my means, but you wouldn't consider that moral, would you?

Only an objective moral system is distinguishable from having no morals at all. Luckily, all those of us that aren't psychopaths are born with an innate, objective morality, guided by our consciences. There's a simple moral principle that corresponds to that. It's called "The Non-Aggression Principle". The idea is that aggressing against people is immoral.