You said: "There is a difference between cursing and using foul language." But the Hadith says to neither curse nor speak foul language.
examples:
[Salim ibn 'Abdullah said, "I never heard 'Abdullah ever curse anything." Salim mentioned that 'Abdullah ibn 'Umar said, "It is not fitting for a believer to be a curser."]
['Abdullah reported that the Prophet, may Allah bless him and grant him peace, said, "A believer is not a defamer nor a curser nor coarse nor obscene."]
[Abu'd-Darda' reported that the Prophet, may Allah bless him and grant him peace, said, "Those who curse will be neither witnesses nor intercessors on the Day of Rising."]
[Abu Hurayra reported that the Prophet, may Allah bless him and grant him peace, said, "The true believer must not be a curser."]
You said: "Thirdly, we only preform lana on people that are for sure going to hell (Yazid, mauwiya, etc)." I love this "etc" of yours.
You do not know for a fact anyone is going to hell, you do not know if YOU yourself are going to heaven so how arrogant of YOU to condemn others when you don't know your destination. Any person might have repented and forgiven by Allah and that is the Ghayb you do not know.
Then you quote this verse:
{If a man kills a believer intentionally, his recompense is Hell, to abide therein (For ever): And the wrath and the curse of Allah are upon him, and a dreadful penalty is prepared for him.}
Al-Qur’an 4:93, translation by Yusufali
So if Allah sent his curse on someone, this is a permission for you to start cursing specific individuals? Who ordered you to curse?
We know for a fact that certain individuals were cursed like Pharaoh and abu Lahab and the like, never did I recall Rasul-Allah (saw) ordering us to curse them. Even though we know for a FACT they're cursed.
If certain people get angry and curse, Allah will forgive them because they were in a moment of anger, just like Allah will excuse an angry person for using foul language, but no one was ordered to curse as a Sunnah.