First of all, wrong treatment of some shiies is not necessarily the meaning of right Islamic religious. And these wrong beliefs should not be considered as shiism fault.
On the other hand, we are damning those who have been damn by God as well. In Koran who has been cruel to prophet and his family, like unbelievers, brutal, evil, phare , Abu lahab and those martyred imam Hussein and his apostles and especially who bothered prophet and his daughter, are apparently damned by God.
The instance could be 57 verse of Al-ahzab chapter which says: who are annoying God and his prophet will be damn by them. Thus, who ignored Imam Alis right, to be the leader of Muslims, are ignorable to be damned.
In fact, not only do we insult those who hold affection of Prophet and imams in their heart but also we believe that they are our brothers and it is our duty to tell them the truth in peace. undoubtedly, if these brothers Knew what false apostles of prophet did to him, they would have damned those either.