Some websites claim, that Ali (a.) declared cursing in public a sin. Therefore one would go to hell if one did so. This is a statement taken out of context.

Harith ibn Hasirah reported, that Abdullah ibn Sharik said: ,,Hajar ibn Adi and Amr ibn Hamiq went into the field while announcing the renunciation and cursing of the people of the Levant. Thereupon Ali (a.) sent a message to the two of them: ﴾Leave that, which has come to me about you.﴿ Then they came before him and said: ﴾O Commander of the Faithful, are we both not right?﴿ He said: ﴾Yes.﴿ They said: ﴾Are they not wrong?﴿ He said: ﴾Yes.﴿ They said: ﴾But why did you forbid us from reviling them?﴿ He said: ﴾It is abhorrent (makruh) for you to revile, curse, and renounce much, but when you describe their bad actions, whereupon you say: ,,They behaved such-and-such and their actions are such-and-such.” Then that would be a more appropriate statement and most admirable for you to justify and would you instead of cursing and renouncing them say: ,,God, keep their blood and our blood from being shed and make peace between them and us and lead them out of their misguidance, so that those of them may recognize the right who did not recognize it and those of them may turn away from the misguidance and enmity, who were inclined to it.” Then that would be better for me and better for you.﴿ Then they said: ﴾O Commander of the Faithful, we accept your reminder and are restrained by your discipline.﴿” [Al-Mustadrak of An-Nuri, Volume 12, page 306 – 307, Hadith 3]

نصر بن مزاحم في كتاب صفين عن عمر بن سعد، عن عبد الرحمن، عن الحارث بن حصيرة، عن عبد الله بن شريك، قال: خرج حجر ابن عدي وعمرو بن الحمق يظهران البراءة واللعن من أهل الشام، فأرسل إليهما علي عليه السلام: ان كفا عما يبلغني عنكما فأتياه فقالا: يا أمير المؤمنين، السنا محقين؟ قال: بلى قالا: أو ليسوا مبطلين؟ قال: بلى قالا: فلم منعتنا عن شمتهم قال: كرهت لكم ان تكونوا لعانين شتامين يشهدون ويتبرؤون، ولكن لو وصفتم مساوئ اعمالهم، فقلتم: من سيرتهم كذا وكذا، ومن عملهم كذا وكذا، كان أصوب في القول وأبلغ في العذر وقلتم مكان لعنكم إياهم وبراءتكم منهم: اللهم احقن دماءنا ودماءهم، وأصلح ذات بيننا وبينهم، واهدهم من ضلالتهم، حتى يعرف الحق منهم من جهله، ويرعوي عن الغي والعدوان من لهج به كان هذا أحب إلي وخيرا لكم فقالا: يا أمير المؤمنين، نقبل عظتك ونتأدب بأدبك

Summary:

The individuals are correct according to the Imam (a.)

The Imam (a.) does not declare their action forbidden (Haram)

But it is abhorrent (makruh) to him

He prefers (Mustahab), that they refrain from it

It is about reviling and the guidance of the living and not about those who are dead and bound to hellfire (Abu bakr, Umar, Yazeed etc.)

Nevertheless there is no question of sin (Ithm)

The Imam (a.) curses himself [Read here!]