Update variable value via add_filter

Filters only allow you to directly modify the first value passed to them. In your case, that’s null. If you shifted view_slug up a place, it would be available to filters using your events_view_html tag.

add_filter( 'events_view_html', 'my_callback', 10, 3); // passing 3 args to the callback, assuming you need them for something.

function my_callback( $view_slug, $query, $context ) {
// Do whatever you need to do to $view_slug
// $query and $context can be read but not changed
return $view_slug;
}