When you make it clear that you are making a personal judgement. You can clearly represent non-biased* history while also stating that e.g. the Hunger Plan of the Nazis in the East was absolutely a horrible thing. The problem arises when you mix morality and research, or interpret sources in a moralizing way without stating so. E.g. for the longest time research about Alexander the Great would not be complete without a moral assesement from a precursor for Christ to an evil Commie force-melting Cultures and everything in between.

The thing is, you can write most stuff into your work as long as your clearly state its intent or that it is your personal opinion. But mostly people (and other historians) aren't really interested into your personal opinion on most historic subjects. You are detracting from your work with uninteresting and irrelevant stuff. Opinions today mostly come into play where the judgement of a given thing is open to debate and not a settled thing. E.g. politicized history (Archeology in the NS-state) or rather recent things (Vietnam war for decades or Afghanistan war today).

*Leaving out that everyone has bias etc. The usual discussion.

Another consideration is the scale of your statement. There should never be a situation where a historian uses one quality to sum up a singe entity, especially at the scale of a person's entire career. This goes against the detailed, critical narratives that historians aim for. It's acceptable to say that a certain one of Napoleon's policies was good or bad for France's economy. It relates a very specific event to a defined scale. Likewise, there's a definite difference between "Hitler was bad" and "Hitler's regime created numerous human rights abuses of a large scale." This is not only good historical practice, but good general writing practice as well.

It depends on the subject, I think. Recently, I took two different classes: on on the early Cold War, and one on Ireland and India and the British Empire. The interesting thing about each of those subjects is that both went through a kind of historical evolution; they had historians in support of one side at first, and then some time later other historians came around in defense of the other side, and now things are more or less historiographical. However, other subjects are really cut and dry and straightforward (not many, mind you).

Overall, I think nowadays a historian's job is much like a scientist in that you have to present the facts as clearly and as un-biased as possible, whether you like the results or not. And obviously, a historian can have opinions and biases about particular events, but the goal is to not really let those seep into your professional work.

However, when presenting a new idea on a topic (off the top of my head, I can think of a few essays I've read on the feminist view of certain historical topics that have never really been researched before) it is harder to present the point you're trying to get across without being biased, but it is possible if you do it properly.

Edit: I think it also deals with the age old question of "what is bad?" and "what is good?" It's all about perspective. So if you're reading the history of the conquest of Mexico, it's pretty clear (and goes without saying) that the conquest was bad for the Aztecs, and good for Spain.