As-salaamu `alaykum warahmatullaahi wabarakaatuh.
In Ibn Jawzee's Talbees Iblees (The Devil's Deception), I believe it mentions that Yazeed didn't order for Husayn's killing, may Allaah be pleased with him. It also states that he was disgusted upon seeing the site of his head, disgusted with the fact that he'd been killed, or at least disgusted with the manner in which he was killed. Of course, we know that the people who have gone to extremes with respects to Ahl ul-Bayt, curse and slander not only Yazeed, but Mu`aawiyyah, may Allaah be pleased with him, and a number of companions.
However, it is worth mentioning that I came across some statements of the scholars (Ibn Taymiyyah being one of them I believe), in which it is said that ahl us-Sunnah do not curse nor praise Yazeed Ibn Mu`aawiyyah, hence the approach we take is a balanced one...
If anyone has anything supporting this, please mention it.
Jazaak Allaahu khayr.
Was-salaam