documentclass[a4paper,12pt,headsepline,dvipsnames,bibliography=totocnumbered]{scrartcl}

usepackage{tikz}
usepackage{tikz-qtree}

usetikzlibrary{trees}

begin{document}

    tikzset{
        attackTree/.style={
            auto,
            font=small,
            edge from parent fork down,
            level distance=2.5cm,
            every node/.style=
            {   rectangle,
                minimum height=15mm,
                minimum width=30mm,
                draw=black,
                align=center,
                text depth = 0pt
            },
            edge from parent/.style=
            {   draw=black}
        },
        treeOperator/.style={
            minimum width=0,
            minimum height=0,
            draw=none
        }
    }

    begin{tikzpicture}[
        attackTree,
        sibling distance=35mm
        ]
        node {Open safe}
        child{
            node {Pick lock}
        }
        child{node {Learn combo}
            child{node {Find written\combo}}
            child{node {Get combo\from target}
                child{node{Threaten}}
                child{node{Blackmail}}
                child{node{Eavesdrop}
                    child{node(listen){Listen to\conversation}}
                    child{node(state){Get target to\state combo}}
                }
                child{node{Bribe}}
            }
        }
        child{node {Cut open safe}}
        child{node {Install\improperly}};

        draw[draw=none] (listen) -- (state) node[treeOperator,midway] {$wedge$};
    end{tikzpicture}
end{document}