Witness in murder trial

I’m not sure if this is the right place to ask this. I am testifying at a preliminary hearing for a murder trial and I’m scared. Do defendants ever retaliate? Now that it’s getting closer the idea of seeing this person and him seeing me and knowing/remembering my name and face is kind of freaking me out. My understanding is that he will be in prison for a long time but crazy things happen in the legal system and so I’m scared he’ll he let off somehow or there will be a situation where if he is sentenced for a long time could he contact someone on the outside to do something. Is that something that happens?