Right Speech & Precepts

The precept for lay people is only about refraining from false or dishonest speech. The precept is not about 'Right Speech' (which includes pleasant, cordial & beneficial speech). Therefore, the precepts allows lay people to criticise people, such as criticizing bad monks but monks are generally not allowed to criticise laypeople because they generally must practise metta.

Regardless, the Dhamma allows criticism in speech, as shown below:

Potaliya, four kinds of people exist and can be found in the world. What four kinds? The four kinds are:

Some people blame those who should be blamed, according to the truth, at the proper time, but do not praise those who should be praised, according to the truth, at the proper time.

Some people praise those who should be praised, according to the truth, at the proper time, but do not blame those who should be blamed, according to the truth, at the proper time.

Some people do not blame those who should be blamed, according to the truth, at the proper time, and do not praise those who should be praised, according to the truth, at the proper time.

Some people blame those who should be blamed, according to the truth, at the proper time, and praise those who should be praised, according to the truth, at the proper time.

Potaliya, these four kinds of people exist and can be found in the world. Of these four kinds of people, that kind should be the most fair and right, the most refined, to you?

Venerable Lord Gotama, of all those four kinds of people, the kind of person who does not blame those who should be blamed, according to the truth, at the proper time, and does not praise those who should be praised, according to the truth, at the proper time; is the kind of person who is the most beautiful and refined to me. What is the reason for this? Because this is fair and right with upekkha (equanimity).

Potaliya, of all those four kinds of people, whichever kind of person blames those who should be blamed, according to the truth, at the proper time, and praises those who should be praised, according to the truth, at the proper time; this kind of person is the most beautiful and refined of these four kinds of people. What is the reason for this? It is fair and right because such a one knows the right time in those circumstances.

AN 4.100

Absolutely , you can criticise someone constructively. Buddhism in no way teaches us to be pushovers. However, the Buddha mentioned from the (AN V (From The Patimokkha, Ñanamoli Thera, trans.)) Source : https://www.accesstoinsight.org/ptf/dhamma/sacca/sacca4/samma-vaca/index.html

In order for your actions to be skillfull you have to admonish the person hurting you or others very skillfully.

How to admonish another skillfully

"O bhikkhus, a bhikkhu who desires to admonish another should do so after investigating five conditions in himself and after establishing five other conditions in himself. What are the five conditions which he should investigate in himself?

[1] "Am I one who practices purity in bodily action, flawless and untainted...?

[2] "Am I one who practices purity in speech, flawless and untainted...?

[3] "Is the heart of goodwill, free from malice, established in me towards fellow-farers in the holy life...?

[4] "Am I or am I not one who has heard much, who bears in mind what he has heard, who stores up what he has heard? Those teachings which are good alike in their beginning, middle, and ending, proclaiming perfectly the spirit and the letter of the utterly purified holy life — have such teachings been much heard by me, borne in mind, practiced in speech, pondered in the heart and rightly penetrated by insight...?

[5] "Are the Patimokkhas [rules of conduct for monks and nuns] in full thoroughly learned by heart, well-analyzed with thorough knowledge of their meanings, clearly divided sutta by sutta and known in minute detail by me...?

"These five conditions must be investigated in himself.

"And what other five conditions must be established in himself?

[1] "Do I speak at the right time, or not?

[2] "Do I speak of facts, or not?

[3] "Do I speak gently or harshly?

[4] "Do I speak profitable words or not?

[5] "Do I speak with a kindly heart, or inwardly malicious?

"O bhikkhus, these five conditions are to be investigated in himself and the latter five established in himself by a bhikkhu who desires to admonish another."

In short , investigate whether you are doing it out of anger our out of a pure intention to protect yourself and the other person as well .Make sure that it is true that the other person is indeed gossiping about you and this is not just your assumption . And then ask yourself whether it is beneficial, to all concerned.If all these three conditions are fullfilled then pick a right time to speak it.

1. You can criticize if it is beneficial to you and/or others, but you should not get angry.
2. You can advice the person not to do so, if you think he will listen. In any case, be mindful of the concern about your ego. You cannot really stop people from talking whatever they like behind your back. Be mindful of the anger arising in you. Do either Metta or Satipattana meditation to deal with anger.
3. Intention takes precedence always. It's never the act. Stealing is wrong because it is caused by greed and delusion. Regarding sliced bread, taking something is ok if the owner does not mind you taking it. It does not break the precept. You can prevent someone from hurting another by blocking his path or holding him tight. But hurting intentionally usually involves aversion. It's up to you to check if hate/aversion creeps in.

According to the Dharma, is it allowed to criticize someone in a constructive manner?

As per Buddha in Pali Canon, it is out duty to point out someone's mistakes. However, Buddha also explained how arguments come from attachment to our views. What looks like mistake to us can be right with the information they have. Buddha in Pali Canon explained that when two people have a disagreement about how things work, they should try to discuss and merge their two perspectives into one, instead of trying to argue which one is correct. This combined merged perspective will be more robust and more useful to both.

If someone gossips about me how do I deal with it? Compassion and kindness aside, sometimes it is necessary to be assertive. One can only be compassionate if one is healthy inside in that moment in time.

Based on what I saw in Pali Canon and other texts and on what my teachers explained to me, I'm pretty sure that the Buddhist approach is NOT to try to argue with gossips, but to keep showing who you really are in action. If you are consistent with manifesting your values in your actions, people will see that, and any gossips that come from invalid or partial information will eventually fade away.

Intention is a little bit more important than the act itself...

Correct, in Mahayana Buddhism, and as explained by my teacher, your hidden agenda in your mind is what really matters. If you act with egoistic agenda, even the best action will be contaminated, so that's bad karma - even if it does not break any rules and succeeds short-term. If you act with selfless agenda, even controversial acts will be good, as long as they are performed with wisdom of understanding the consequences for others. But yes, in terms of your feeling good and not having doubts about your choices, as long as your intention was selfless and you did not harm others, your karma is good, so you should feel good and have no doubts.