Since you ask: your first paragraph is a pretty common confusion that we've seen many times before. It's entirely reasonable for you to ask, and this is the right place to ask it, but it's not very fun for us to answer it again. The second paragraph is strange, transparently wrong, and a little bit offensive; I think this is where the downvotes are coming from.

Since nobody has any reason to reduce suffering other than 'I want to' / 'I feel so'

This is tautologically true, but I don't think it's interesting. Nobody has any reason to eat ice cream other than "I want to," but even after you explain where that urge comes from, I still want to eat ice cream.

it is kind of hypocritical to say that "utilitarianism is for greater good"

Assuming for the sake of argument that this is true: so what? Ignore the motivations; does utilitarianism actually serve the actual greater good? Does the answer change depending on the altruist's mental state? From the consequentialist perspective, hypocrisy isn't even relevant.

Therefore when you coerce other people or kill one to save three, you do this not because of "greater good" but because you like to coerce and kill.

These aren't things that happen, and I have no idea where you're getting this. It made me wonder if you're trolling, but I think that's unlikely because you seem to be acting in good faith elsewhere in the thread.

If we liked to coerce and kill, we would spend more time coercing and killing, and less time on this altruism thing. None of the people you're addressing has ever done anything more coercive than writing a blog post, never mind killing one person to save three. If I ever had to do that, I would feel terrible.

I hope that helps!