ALSO, I was thinking about this thread while I was offline, and I noticed something interesting about all this:
The reason why people are "seeing the worst" in the person who sees the worst in others is entirely because of the way the question was phrased. Like Brovo said, we don't know enough. More specifically, the only thing we know about this hypothetical person is that, like you said, they "see the worst in others". Therefore, this hypothetical starts off with us actually thinking that about a person -- so of course most of the answers will be negative, because it assumes that we're thinking negatively about them to begin with, as I would think that "always seeing the worst in others" would generally be considered a negative trait, and hard to not associate with pessimism.
If this were a real person that we just so happened to encounter, it's possible that we wouldn't even describe them as "seeing the worst in others". There will surely be other character traits to pick up on -- it's possible that we might learn other things about them that might explain why they have such a worldview, or it's possible that we won't even see that in them at all -- we could be seeing other, more positive things. Therefore, even if this hypothetical person could fit the description of "seeing the worst in others", it's possible that many of us wouldn't see them that way.
The way the question is phrased, though, we only know of a single, negative trait of this person. So, of course most answers will make it seem like we're assuming the worst about them -- because the only thing we know about them is negative. And, unlike in the case of a real person, with whom we can at least try to focus on other things if we want to be optimistic -- there is literally no other character trait to focus on, here. We're seeing the worst because the worst is all you've allowed us to see.
Personally, my answer had more of a positive spin on it, but that's only because I prefer to give people the benefit-of-the-doubt and assume that there could be other reasons why they're acting a certain way. But, that got me thinking -- anything I could speculate about why this hypothetical person could be acting a certain way is all just wishful thinking on my part, which isn't too different from what I tend to do in reality, I suppose, but, at least then, I have a bit more background info on which to base some of those assumptions. Even if it's a complete stranger, I could still assume some things just based on the situation they're in. In your hypothetical, though, there's nothing to work off of, which is probably why I remember not being able to really think of any examples when I gave my initial answer. Point is, my answer still revolved around trying to justify what would normally be considered a negative behavior. That was still my starting point, and the only way I could possibly switch things into a positive light would be to make up possible explanations for them being that way.
Heck, you didn't even say whether or not we know this person well or not. If it's someone we do know well, then we probably know a lot more about them, and it would certainly be possible to more easily see more positive traits, or at least understand their actions better. And if it's a complete stranger, then, how do we even know that they see things this way? Is that an assumption that we're basing off of them making one or two negative comments about other people? Either way, the only way we could even see this person as "seeing the worst in others" is if we're framing them in a negative light to begin with, which, yeah, would mean that we are also probably seeing the worst in them. It's no wonder that a lot of our answers reflect that sort of mentality, then -- since the scenario, as you described it, would assume that we're all focusing on the negative in this person to begin with.