Why when some people are confronted with something they did wrong they just call the other person negative ?

They are trying to side step, to avoid, to divert and redirect the focus. Don't let them. 

"You're negative; you're toxic; I don't have to deal with your negativity!"

"Be that as it may, the issue here is your whoring around, your thievery, your back stabbing. Stop trying to evade the issue." 

That doesn't get them out of it, it's a passive aggressive ploy to throw you off the track.  Generally when one behaves in this manner, they are guilty, and this is how they deal with their guilt.