Shouldn't Catholics practice kindness?
Is it a sin to malign, smear, slander, libel, character-assassinate, etc?

2477 Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury.277 He becomes guilty:
- of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
- of detraction who, without objectively valid reason, discloses another's faults and failings to persons who did not know them;278
- of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them.

2478 To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor's thoughts, words, and deeds in a favorable way:

Every good Christian ought to be more ready to give a favorable interpretation to another's statement than to condemn it. But if he cannot do so, let him ask how the other understands it. and if the latter understands it badly, let the former correct him with love. If that does not suffice, let the Christian try all suitable ways to bring the other to a correct interpretation so that he may be saved.279

2479 Detraction and calumny destroy the reputation and honor of one's neighbor. Honor is the social witness given to human dignity, and everyone enjoys a natural right to the honor of his name and reputation and to respect. Thus, detraction and calumny offend against the virtues of justice and charity.

2480 Every word or attitude is forbidden which by flattery, adulation, or complaisance encourages and confirms another in malicious acts and perverse conduct. Adulation is a grave fault if it makes one an accomplice in another's vices or grave sins. Neither the desire to be of service nor friendship justifies duplicitous speech. Adulation is a venial sin when it only seeks to be agreeable, to avoid evil, to meet a need, or to obtain legitimate advantages.

The things that are said about Pope Francis appear to me to be very unkind. Shouldn't we (by which I mean Christians) try to see people in the best possible light? At what point is it okay not to see people in the best possible light, e.g. Putin, Trump, whoever?

Is poor mental health an acceptable reason for not being kind or not loving your neighbour?

1734 Freedom makes man responsible for his acts to the extent that they are voluntary. Progress in virtue, knowledge of the good, and ascesis enhance the mastery of the will over its acts.

1735 Imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors.

1736 Every act directly willed is imputable to its author:

There are some people I currently feel very angry towards, they are not people I know personally, I've never met them, they all belong to a government organisation which I believe, which I know is causing great harm and stress to vulnerable people unnecessarily. Cruelty and injustice make me angry. How can I love heartless, uncaring people? Only this is a Catholic message board I'd use much stronger language. How can you love cruel people without condoning what they are doing?

I don't know how. In the meantime I'll just focus on being loving and kind towards all the people who are not cruel and heartless.

(If anyone in the UK is reading this, I'm referring to Capita and the cruel and heartless rich politicians responsible for PIP, who are making physically and mentally ill people more ill.)

"Nevertheless, nationalism is a dangerous error, and he should distance himself from supporting it"

aively.

Love is not always nice. It may require harsh language and at times rebuke. But that said, I think we can agree we tend to use that to justify harsh language and abuse more often than is actually justified by love.

Of course libel and slander are sins both against charity and truth. But even detraction (saying something bad about someone else that is true) is a sin, unless done out of charity for a good reason (for example, telling my sister her boyfriend is cheating on her, in order to spare her from an unhealthy relationship)

Public figures are hardcases here. We should care about whether, e.g., a politician is corrupt or virtuous. I think too we can make a distinction between interpreting a person in the best possible light, and recognizing the actual perception people have of that person.

So with Trump, as to his person, I should, as far as I can truthfully, interpret his actions and statements charitably, and not assume the worst possible motives. But I should recognize that, regardless of his intentions or his intended meaning, actions and statements of his will have ill effects and be understood in worse lights. So if Trump says something that could be taken as endorsing white nationalism, but could be understood differently, then in my judgment of his person, I should assume the more benign interpretation, but I must recognize that his words would also promote, intentionally or not, white nationalism and I should denounce that. So if he said "I am a nationalist" I can both truthfully say "he probably means only that he is patriotic" and "Nevertheless, nationalism is a dangerous error, and he should distance himself from supporting it"

It can be a fine line at times. Also, sometimes a person makes manifest their intention in an uncontrovertible manner. We should judge charitably, not naively.