Too much harted, that itself violate one of basic of Islam, is hatred is Haraam, and that result in breaking the unity apart, relationship like family apart.
If they can't provide the reasons to back with, as their not being convinced with Zakir Naik. Then, better to keep your opinions for your own good. ALLAAH (SWT) will ask us why we didn't contribute anything to Islam, and yet dared to criticize someone else contribution? ALLAAH (SWT) knows this man better than all of us, and better than himself. ALLAAH (SWT) will ask everyone as for everyone contribution for Islam will be questioned definitely. Try to judge less, and try to contribute meaningful for Islam. We are not allowed to judge, criticize, and talk behind people back at all.
We, being Muslim, least should do is appreciate their contributions for Islam, and if we have legitimate reasons to question their contributions, feel free to do so along with the hard evidence to back with. Questioning like this, without proper evidence to back up is seriously unacceptable.
^ Abdullah,
The basic of Muslim procedure is we don't judge, hate, criticize, complain. That is seriously forbidden. No one is perfect, and there will be weak point of everyone, and Zakir Naik is no less than human either. Try to analyze his contributions more than his character and his demands. That's his business and ALLAAH (SWT), and meanwhile, we can listen to Zakir lectures if we'd like, and if you don't want to, feel free not to, and that's also your right too. We Muslim people must not hate, judge, and complain at all. We are Human, and bound to make such mistakes, and therefore, we always have opportunities to repent and ask forgiveness from ALLAAH (SWT), in order to cleanse our soul again if we continue to pray five times a day. Remember, ALLAAH (SWT) the most merciful ever!