Mmm, I'm not sure that's entirely the same though. I think you're jumping to ideologies of groups that people subscribe to, as opposed to "actions of few". For some things, I think you can reasonably make some assumptions on groups of people based by who or what they follow.

Let's say you have PETA. It's reasonable to believe that a large proportion of those followers would be vegan, simply because of the ideologies expressed by that group they adhere to (PETA promote being a vegan). Now let's say that after a rally, a few of them were arrested for stealing donations or something. You wouldn't assume a huge proportion of PETA supporters were thieves, simply because a few of them were arrested for committing those acts.

An extreme example might be the KKK. You can reasonably assume that the essentially everyone in the KKK hate African-Americans because that is an ideology they follow. However, let's say a handful of KKK members also committed sexual acts against children. As disgusting as these people are for their racial beliefs, you couldn't reasonably assume that the majority of KKK were pedophiles because a few of them committed such acts.

With the Police, you can reasonably assume that the vast majority of officers want to serve and protect their community, because that is an ideology Police subscribe to. If a few officers discriminate against minorities, you can't assume that the vast majority of officers also discriminate in that way because not all of them do.

As for Donald Trump, he is a (potential) politician with a following. Aside from their personality/history, you like a politician primarily for their beliefs and what they stand for and therefore tend to agree with most of their views. Now Trump has expressed bigoted views countless times and has even built popular points around them (i.e. Mexico wall, Muslims being singled out at airports etc etc). He has made it a somewhat significant ideology in his campaign and therefore it's not too unreasonable to believe a significant proportion of his followers would be bigoted to some degree because a large proportion would support these bigoted beliefs of his. 

So yeah. I think when it comes to Trump, I'm not sure the saying still applies because it's not so much the actions of a few anymore, it's more so the ideology they are supporting by supporting Trump. In general, the concept of not judging a whole group of people based on the actions of a few is essentially a guideline in the sense that I'm sure there are exceptions here and there. It can also especially be relevant when it comes to racial stereotypes too, for example.