May Allah curse those who don't believe cursing him is halal. He is the scum of the Earth and a wretched dog.
Don't curse those who abstain from cursing, not everybody went and researched the event then concluded what you did. Some also follow narrations that advise Muslims not to curse one another. Some do not know whether he repented or not. If a person prefers to abstain and keep his tongue purified then good for him, we were not ordered by any religious text to curse a specific person, if we did we'd be cursing Pharaoh 24/7 whose crime is a million times worse than Yazid.
Yazid's irresponsible decisions led to the death of a great man, he wronged al-Husayn bin `Ali and oppressed him.
Pharaoh claimed Godhood and wronged God Himself! Yet Musa (as) was ordered to speak to him softly and calmly.
Point is, those who are certain of his crimes rightfully curse him. Those who are uncertain or unsure, refrain.
Dear brother, know that even recent scientific studies show that every negative statement you make will affect your psychological condition. Every foul word harms the person who utters it more than it harms others. Keeping the soul pure and healthy requires that you abandon certain matters, grudges, hatred toward fellow man etc... and leave all of it up to God. The wisest of people don't curse the person but curse the action, such as saying "May God deliver justice to the oppressed and guide the oppressors." Or "May God judge every tyrant on the day of judgement." etc...
Again, I personally don't have any issues with those who curse him for his terrible deeds.