Color background for chapter titles which stretches horizontally to the margins

Here a suggestion using mdframed. I also removed the package tocloft which isn't needed.

\documentclass[12pt,a4paper,
%   Add lists to TOC
listof=numbered,numbers=noenddot,version=first,
]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\inputencoding{utf8}
\usepackage{helvet}
\usepackage{mathpazo}
\usepackage{setspace, lipsum}
\usepackage{xcolor}
\usepackage{mdframed}
\mdfdefinestyle{chapter}{rightmargin=-3cm,innerrightmargin=3cm,backgroundcolor=red,hidealllines=true}
\addtokomafont{chapter}{\color{white}}
\makeatletter
\renewcommand*{\@@makechapterhead}[1]{%
  \chapterheadstartvskip
  \begin{mdframed}[style=chapter]
    \setlength{\parindent}{\z@}\setlength{\parfillskip}{\fill}%
    \normalfont\sectfont\nobreak\size@chapter{}%
      \if@chapterprefix
      \let\@tempa\raggedsection
    \else
      \let\@tempa\@hangfrom
    \fi
    \@tempa{\ifnum \c@secnumdepth >\m@ne%
          \if@chapterprefix
            \expandafter\size@chapterprefix
          \else
            \expandafter\size@chapter
          \fi
          {\chapterformat}%
          \if@chapterprefix
            \size@chapterprefix{}\endgraf\nobreak\vskip.5\baselineskip
          \fi
      \fi
    }%
    {\raggedsection \interlinepenalty \@M \size@chapter{#1}\par}%
  \end{mdframed}
  \nobreak\chapterheadendvskip
}
\renewcommand*{\@@makeschapterhead}[1]{%
  \chapterheadstartvskip%
  \begin{mdframed}[style=chapter]
  {\normalfont\sectfont\nobreak\size@chapter{}%
    \setlength{\parindent}{\z@}\setlength{\parfillskip}{\fill}%
    \raggedsection \interlinepenalty \@M \size@chapter{#1}\par}%
  \end{mdframed}%
  \nobreak\chapterheadendvskip%
}
\makeatother


\begin{document}
%   TOC
\tableofcontents
\pagebreak

\chapter{Introduction}
\lipsum[1]
\chapter{Main Part}
\lipsum[2]
\chapter{Conclusion Conclusion Conclusion Conclusion Conclusion Conclusion Conclusion Conclusion Conclusion }
\lipsum[3]

\end{document}