There is this guy which liked me in the past and I stopped having communication with him because I found out he was treating people negatively. I went to a same party as him and I caught them staring at me a lot and I think he was drunk. Why was he staring at me?
He may have been staring at you because he found you attractive.
Being that he was drunk, he may not have necessarily remembered who you were.
If the both of you aren’t all that acquainted and haven’t spoken to each other in a while, it is even more likely that at that moment while he was drunk and staring, he either couldn’t remember you or thought that you looked familiar but couldn’t quite remember why.