set css border to end in a 90 instead of a 45 degree angle

You can do this with box-shadow.

Demo:

Output:

CSS:

#borders {
    border-bottom: 20px solid black;  
    box-shadow: 20px 0 0 0 red;
    height: 150px;
    margin: 30px;
    width: 150px;
}

HTML:

<div id="borders"></div>