Muawiya reached an agreement with Jada bint al-Ash’ath bin al Qais, namely 100,000 dirhams if she poisons Imam Hasan.

Sunni References:
-Tadkhirat au Khawwas, p192
-Allamah Zamakshari, Rabi ul Abrar, V4, p208
-Allamah Ibn Abdul Barr, al-Istiab
-Shawahid un Nubuwwa, p.303

"And whoever kills a believer intentionally, his recompense shall be hell, he shall abide therein and God’s wrath shall be on him and his curse, and is prepared for him a great torment”
(Surah Nisa:93)

But killers of grandsons of Prophet (pbuh) are ameer ul momineen!!

Hazrat Ayesha said: "Muawiya you killed Hujr and his associates, By Allah! The Prophet told me ‘In the ditch of Adra seven men will be killed, due to this all the skies and Allah will be upset”.

Sunni Reference: Tarikh ibn Asakir, V.12, p.227

A contemporary Hanafi scholar, Mufti Ghulam Rasul, wrote in his book Subeh al-Sadiq, p.93/94 , that Hujr ibn Adi who was companion of Prophet (pbuh) and his companions were killed because they refused to curse Ali ibn Abi Talib.

In addition to the things mentioned above, Muawiya fought various battles against Imam Ali and never accepted his caliphate.
Later on he appointed his son Yazid as the ruler over the Muslims after him, who was the pure evil. (Imam Hanbal said that yazid died an infidel)!

In the light of verse of the Holy Quran mentioned above in this thread, the Shia seem right to curse Muawiya, by asking Allah to deprive him of His mercy. Why can't our Sunni brothers agree to this? Why they want shias to respect him as he clearly was an enemy of ahlulbayt?!?!?