Why is the killer in Scream trying to kill Sidney?

Are you asking about Scream or Scream II? In the original one, the Skeet Ulrich guy did it because of Sydney's mom and his dad hooking up causing his mom to abandon him but then only kind of, because in the end it was more like he did it just because he was the dominant psychopath sadist of their duo, while the other guy seemed like he did it because he was a submissive with a bit of a bromance crush on Skeet Ulrich. After all was said and done in those crazy kitchen scene monologues, it seemed like they did it for sh*ts and grins because they were mentally deranged more than any other reason, I felt. They just kind of used the Sydney/Sydney's mom story as a jumping off point, as an inspiration to their homicidal mania.

Then, the subsequent movies/stories were based on groupies/followers/copy cats of the first story that had gained notoriety and a cult status because of that reporter's, played by Monica from Friends, reporting of the events. Sydney became a target less because of her mom after that and more because she was the target of the first round of killings that happened in the first installment. The story of her mom turned out to be more of a catalyst than a reason. Then, the subsequent killings were catalyzed by the first, so the story of her mom just became more and more tangential. The story of her mom was kind of used as a red herring in the following one just like it was in the first, to make viewers think that the story was about that in order to twist the plot when it really turned out to be about something else, which was primarily plain old homicidal mania looking for any old reason without too much of a reason. That's my take on it anyway.

it's because her mom helped his(the killer) dad cheat on his mom which drove his mother to leave him; the rest is just really stupid, I think. they're doing it to keep the story going. (still good movies though)

Well there wouldn't be a show ha ha