1) On social media platforms, like Youtube, FB, IG, people upload short videos or Islamic posts, if some people of other sects comment like shia kafir and bad things about us, what should we do? should we stay quite or we can say something unlawful to them coz its hard to control after seeing some negative comments on our shia community..

If they say bad about us, will we be rewarded for it in akhira?

by

If you believe that you can change their mind then you can talk to them
politely by showing them the manners of Ahlulbayt. If you think they will not listen to you then it’s best to ignore them.

If they attacked you personally yes God will reward you for your patience.