Setting new lines and adding them to a list to be drawn
Do you know about radians?

Math.Cos (and the other trig functions in .Net) are based on radians not degrees.

Math.Cos(90) radians is roughly Math.Cos(5157) in degrees. Which is a bit odd...

There are 2π radians in a circle so 1 degree is π/180. As long as you use the same value the lines will be parallel anyway but I thought it was worth pointing out.

Why do you need the Cos at all?

You have a line from (x1, y1) to (x2, y2) with length L = Math.Sqrt((x1-x2)^2 +(y1-y2)^2).

You know that you need to offset the line by an amount (offset) orthogonal to the current line.

private Line CreateParallelLine(Line target, int offset)
{
    var parallelLine = new Line();
    var xDifference = target.point1.x - target.point2.x;
    var yDifference = target.point1.y - target.point2.y;
    var length = Math.Sqrt(Math.Pow(xDifference, 2) + Math.Pow(yDifference, 2));

    parallelLine.point1.x = target.point1.x - offset * yDifference/length;
    parallelLine.point2.x = target.point2.x - offset * yDifference/length;
    parallelLine.point1.y = target.point1.y + offset * xDifference/length;
    parallelLine.point2.y = target.point2.y + offset * xDifference/length;

    return parallelLine;
}


Edit:

I realised that I've only given you an opportunity to create a single parallel line but I think it's straightforward to call the method more times with different offsets!

You didn't show your Line class/struct but it's worth noting that point1 should be at least PascalCase (Point1) but preferably renamed to something more like: Start and point2 renamed to be End or similar.

As x and y are also public, you should PascalCase them too: X and Y.