We can't even get humans to agree on ethical values, let alone explain them to an AI, so even a perfectly altruistic AI could still be a disaster if it has the wrong value system, or if we don't perfectly understand the consequences of our own values systems when fully enacted.
The OP said:
> I'm not sure I understand how exactly a human-equivalent AGI would be more dangerous than an actual human.
Since an actual human might be out to get me already the question is not motives but capability - should I fear evil AIs more than I fear evil humans? Or an evil human with a friendly leashed AI?
The argument typically revolves around whether or not AI would self-improve and lead to runaway intelligence, but I think that that's a distraction - there are plenty of other ways AI could be dangerous that are easier to persuade people with without running into the that-sounds-like-scifi filter.
I think a more interesting question is "how do we make an intelligence, artificial or human, that isn't potentially dangerous?"
Humans have a bunch of low-level primate aggression that seems to be wired in; we can modulate it (we're largely self-domesticating) but we can't totally get rid of it and more importantly, we can't look at a baby's genome (and nurturing environment) and say "yup, this one's going to grow up to be Hitler", or "this 'un's going to be a pacifist, altruist, and general benefactor".
On the other hand we can probably add a bunch of monitoring code to any AI we develop rather than grow: if nothing else, consider the possibility of simulation runs looking for an empathy deficiency.