Hello,
The button is rotated in the above picture because it rotates when open (and goes back to "straight" when the menu is closed). Not sure if that was obvious, since on your site you are using a menu button that is open by default (so it looks "tilted" until it is closed).
If you didn't want the menu button to rotate, you would need to do so by adding some CSS to your site. First, you would need to give your menu button a custom class. You can do so by selecting the menu button in your Buttonizer editor, clicking on the Style tab, then going to the "Custom ID & Custom Class" section and add a custom class name (like staticmenubutton for example):

Then, you can add this style to your site:
.staticmenubutton i {
transform: translate(-50%, -50%) rotate(0deg) !important;
}
The menu button icon should then not rotate when the menu is open.
Hope that helps!