I have seen the "terror" that Shiv Sena activist/supporters have caused first hand. I have seen them beat up people for celebrating Valentine's Day. I have seen them loot shops.
But I also know that a majority of Hindus do not support them.
My point is that by banning such oragizations we push them to the extreme. We have to involve them in discussions and debates.
SIMI has nothing to offer. People will reject their message. That is how you defeat them.
Shiv Sena was a rising force in the 80s and the 90s. But their message lacked substance. Now they are on the decline. Banning them would have strengthen them. Not because of their message, but because they would gain sympathy from people who otherwise would not support them.
Same situation is with SIMI. Muslims will not even remember their name. They are famous because they are Banned. Their is their biggest claim to fame.
Terrorism is broad range of actions that individuals/groups of individuals indulge in to scare the population. This may include killing people.