Why are the aliens being kept a secret? Dont answer if u thimk there ?

if you think there are none. Im very curious as to why, why kepp it such a big cover up? Well ive some research and ive read that they didnt want to scare everybody, and that some military personels found out the truth and had a nervouse breakdown and some people have committed suicide. I dont understand, whats that scary? Why would the government kill anyone who tries to expose it? That,means,that,the aliens arent friendly?? If they were then we would have known by now? What to you think it is? Or have anything?